Our company puts a GA tracking code on each of our websites (we build websites for companies and then provide GA tracking for the company). One of our clients also has a GA code they want on their site. So we have a site that has two GA codes on it, and when comparing the Google Analytics data for sessions, users, etc. the numbers on each account do not match.
For example, for Jan 1 - Feb 10 '16 their tracking code shows 14,905 sessions, while our tracking code shows 13,605 sessions for the same exact range.
Any idea why the data wouldn't match for the same site? Is sessions tracked the same way for every GA code?