First check that it isn't that two Images are in the same Stack, and that the difference isn't from expanding/collapsing that Stack.
There is no command history in Aperture, but you can sort your Images by the date they were modified. That may help.
The other thing to note is that the number shown in the bottom border of the Browser is _very_ specific: it is a count of the number of Images currently shown. This changes as Stacks are expanded and collapsed, and -- important here perhaps -- does _not_ change under some conditions until the selection changes. (Select five Images in a project of 20 with the Browser filtered to the default "Unrated or better" (# shown = 20). Using the control on the Info tab of the Inspector, change the rating to "rejected" (# shown = 20). Click to select an un-selected Image in the Browser (# shown = 19).
It's important to take into consideration where you got your counts, and what state the Library was in at the time.