Strange session data when using NodeJS, ExpressJS and connect-mongo

In a simple test of the connect-mongo module, I found the session store data (printed back to me via the response) a little strange looking:

{
  "__utma": "95606537.1568267584.1375175452.1375367168.1375378799.6",
  "__utmz": "95606537.1375271524.3.2.utmcsr=google|utmccn=(organic)|utmcmd=organic|utmctr=(not provided)",
  "connect.sid": "s:w5IeMa-4K6MQTZKOudqBkO3k.vaebkLoV3y3tuD6rx47tUhUWZOMOM8WmW3LCUWY5I8I"
}

Can anyone explain to me what the __utma and __utmz key/values are and why they have only popped up once (other tested sessions seem to be free of these).

Here's my simple setup:

var express = require('express');
var MongoStore = require('connect-mongo')(express);

var app = express();

app.use(express.cookieParser());

app.use(express.session({
  secret: "secret",
  store: new MongoStore({
    db:"sessions"
  })
}));

app.use(function(req, res, next){

    res.send(req.cookies);

})

app.listen(3001);

Edit:

Seeing now that I tried hitting the server from different browsers, and that I'm responding with parsed cookie data instead of req.session, I'm guessing the _utmz and _utma aliens are being appended to the cookie through Chrome. Can someone confirm/disconfirm this?