GAS firing MaxScroll with sampling

analytics.js

var _gas = _gas || [];
_gas.push(['_setAccount', 'UA-YYYYYY-Y']); // REPLACE WITH YOUR GA NUMBER
_gas.push(['_setDomainName', '.mydomain.com']); // REPLACE WITH YOUR DOMAIN
_gas.push(['_trackPageview']);
_gas.push(['_gasTrackForms']);
_gas.push(['_gasTrackOutboundLinks']);

// Max Scroll on 20% of the pages
if(Math.random() * 100 < 20) {
  _gas.push(['_gasTrackMaxScroll']);
}

_gas.push(['_gasTrackDownloads']);
_gas.push(['_gasTrackYoutube', {force: true}]);
_gas.push(['_gasTrackVimeo', {force: true}]);
_gas.push(['_gasTrackMailto']);

(function() {
var ga = document.createElement('script');
ga.type = 'text/javascript';
ga.async = true;
ga.src = '/gas.js';
var s = document.getElementsByTagName('script')[0];
s.parentNode.insertBefore(ga, s);
})();