I think you'll find it is the "Smart Grid" doing it's thing, which I love BTW.
To turn it off left click and hold on the resolution selector and uncheck the "Smart grid" option. You can then set your preferred resolution which won't change.
Remember that you can now set a secondary snap. Just hold N down while choosing parameters. You can then call it up by either holding down the N key and then releasing for just a temporary change, or pressing ctrl+shift+N for a permanent change.
I have one set as the smartgrid and the other set to a fixed setting (which depends on my screenset). I think it's great once you used to it. YMMV of course
PRV also has it's own snap setting that can be set and turned on or off by the controls toward the top right of the PRV. The global snap needs to be on for that to work, it's a snap over ride rather than independent of the global snap.
Finally each screenset can have its own snap if you wish but you need to set a variable in the ini file.
IncludeSnapInScreenset=1 HTH.