What are the difference between Revenue vs Product Revenue in Google Analytics?

I have seen in GA that Product Revenue is higher than the actual Revenue in standard report. Should it not be the other way? I have seen the measurement example of the purchases from Google - and it does not add up from what i see from the debug and GA report.

Google measurement example:
<script>// Send transaction data with a pageview if available// when the page loads....

What are the difference between Revenue vs Product Revenue in Google Analytics?