I came to notice that sometimes there is a slight difference between the hps values from the on given in the analyze tab vs the ranking tab.
Here you can see an example for my last Guldan kill:
https://www.warcraftlogs.com/reports/WPM3w6cqAtLf8pnv/#type=healing
In analyze i did 499k hps, while in ranking it was 482k hps. I saw the same behavior in the other Guldan logs i checked.