I wouldn't forget working it out, actually - I'd just keep in mind that they're guidelines, and can't be exact. But for most purposes, they'll be pretty close.
To your second question: Elapsed time, mostly.
For instance, I ran an adventure where the PCs started at 1st-level and were investigating some murders / a fire / a disappeared town councilman. I had a rough plan in my head for how the adventure would go, and at key points (a discovery, a major fight, etc.) they advanced a level.
In terms of actual encounters met and overcome, they were probably well short of the recommended amount, but we didn't get to play very often, so stretching out the levels until they would have levelled normally would have mean that they'd only gain about a single level and a half in a year of playing, and that's far too slow for my liking.
Plus, I like to get out of the low levels faster, rather than slower, and then slow down leveling slightly after they party reaches 5th- or 6th-level or so. Normally, I'd start at 3rd level in PF or 3.5, but I had a bunch of house rules that I wanted the players to get comfortable with before they hit higher levels.