Read that table twice, because it is where most of the surprise in an Apify invoice lives. The memory setting is a reservation, not a measurement. An Actor left on a 4 GB default that never touches more than 900 MB is spending compute units four times faster than one sized to fit — same code, same pages, same speed, four times the compute line.
It also explains why "pages per compute unit" varies so wildly. A static HTML page parsed at 512 MB in half a second and a JavaScript storefront rendered in a headless browser at 4 GB over eight seconds differ by a factor of well over a hundred in what they cost, even though both produce exactly one price.
That is the whole reason the difficulty toggle at the top of this page moves the total more than any other control. It is standing in for a variable — how much machine your targets force you to spend — that you cannot know until you have already run the job.