Nav element is wrapping at 660px, then re-aligns at 720px. Any ideas why this is happening?

 /***************************************
 GENERAL
 ****************************************/

body {
  font-family: 'Open-sans', sans-serif;
}


#wrapper {
  max-width: 940px;
  margin: 0 auto;
  padding: 0 5%;
}

a {
  text-decoration: none;
}

img {
  max-width: 100%;
}

h3 {
  margin: 0 0 1em 0;
}

/***************************************
 HEADING
 ****************************************/

header {
  float: left;
  margin: 0 0 30px 0;
  padding: 5px 0 0 0;
  width: 100%;

}


#logo {
  text-align: center;
  margin: 0;
}

h1 {
  font-family:'Pacifico', cursive;
  margin: 15px 0;
  font-size: 1.75em;
  font-weight: normal;
  line-height: 0.8em;
}

h2 {
  font-family: 'Merriweather', serif;
  font-size: 0.75em;
  margin: -5px 0 0;
  font-weight: normal;
}



/***************************************
 NAVIGATION
 ****************************************/

nav {
  font-family: 'Cutive', serif;
  text-align: center;
  padding: 10px 0;
  margin: 20px 0 0; 
}

nav ul {
  list-style: none;
  margin: 0 10px;
  padding: 0;
  }

nav li {
  display: inline-block;
}

nav a {
  font-weight: 800;
  padding: 15px 10px;
  }


/***************************************
FOOTER
****************************************/

footer {
  font-size: 0.75em;
  text-align: center;
  clear: both;
  padding-top: 50px;
  color: #ccc;
}

.social-icon {
  width: 20px;
  height: 20px;
  margin: 0 5px;
}


/***************************************
 PAGE: PORTFOLIO
 ****************************************/

#gallery {
  margin: 0;
  padding: 0;
  list-style: none;
}

#gallery li {
  float: left;
  width: 45%;
  margin: 2.5%;
  background-color: #f5f5f5;
  color: #bdc3c7;
}

#gallery li a p {
    margin: 0;
    padding: 5%;
    font-size: 0.75em;
    color: #bdc3c7;
}



/***************************************
 PAGE: ABOUT
 ****************************************/

.profile-photo {
  clear: both;
  display: block;
  max-width: 150px;
  margin: 0 auto 30px;
  border-radius: 100%;

}





/***************************************
 PAGE: CONTACT
 ****************************************/

.contact-info {
  list-style: none;
  padding: 0;
  margin: 0;
  font-size: 0.9em;
}

.contact-info a {
  display:block;
  min-height: 20px;
  background-repeat: no-repeat;
  background-size: 20px 20px;
  padding: 0 0 0 30px;
  margin: 0 0 10px;
}

.contact-info li.phone a {
  background-image: url('../img/phone.png')
}

.contact-info li.mail a {
  background-image: url('../img/mail.png')
}

.contact-info li.twitter a {
  background-image: url('../img/twitter.png')
}




 /***************************************
 COLORS
 ****************************************/

/* site body */
body {
  background-color:#fff;
  color: #999;
}

/* dark brown header */
header {
  background: #454740;
  border-color: #748e59;
}

/* nav background on mobile devices */
nav {
  background: #9b8c78; 
}

/* logo text */
h1, h2 {
  color: #fff;
}

/* links */
a {
  color: #454740;
}


/* nav link */
nav a, nav a:visited {
  color: #fff;
}

/* selected nav link */
nav a.selected, nav a:hover {
  color: #fff;
}

@media screen and (min-width: 480px) {

  /***************************************
  CONTACT PAGE: TWO COLUMN LAYOUT
  ****************************************/

  #primary {
    width: 50%;
    float: left;
  }

  #secondary {
    width: 40%;
    float: right;
  }



 /***************************************
 PAGE: PORTFOLIO
 ****************************************/

 #gallery li {
  width: 28.3333%;
  }


 /***************************************
 PAGE: ABOUT
 ****************************************/

 .profile-photo {
   float: left;
   margin: 0 5% 80px 0;
  } 

}


@media screen and (max-width: 480px) {


 /***************************************
 PAGE: PROFILE
 ****************************************/  



    #gallery li:last-child {
      clear: left;
  }

    #gallery li:nth-child(3) {
    clear: left;
  }
}


@media screen and (min-width: 660px) {


  /***************************************
  HEADER
  ****************************************/

  nav {
    background: none;
    float: right;
    font-size: 1.125em;
    margin-right: 5%;
    text-align: right;
    width: 45%;
  }

/* selected nav link */
nav a.selected, nav a:hover {
  color: #9b8c78;
}




  #logo {
    float: left;
    margin-left: 5%;
    text-align: left;
    width: 45%;
  }

  h1 {
    font-size: 2.0em;
  }

  h2 {
    font-size: 0.825em;
    margin-bottom: 20px;
  }

  header {
    border-bottom: 5px solid #9b8c78;
    margin-bottom: 60px;
  }


}