There is nothing worse when preparing for a launch than a song not matching. Unfortunately it happens since LMI only changes small sections of the songs. That said, the industry has come a long way from when Apple grayed out 'illegal' songs or failed to sync them because they were not in the cloud (that is why I quit using an iPod).
To the problem... If you notice the song length is different in your notes than in Google Music (or iTunes), you have to trigger a song refresh. In iTunes you may gave to re-upload, but in Google, if you have the folder mapped you can right-click and select 'Fix Incorrect Match' as shown below.
The key is to do this BEFORE you spend time practicing so you don't waste time - it only takes a few minutes and it can save you!
In this example CX21 track 4 has a whole weighed bridge you would miss
Helpful links if you're Apple and not Google:
If you want to bail on iTunes match (like I did)
If you need fix the song