dcs1955... I'll try to answer your questions...
Because it is the total of tasks that are flagged as valid (validated) during that day... remember, some tasks aren't validated immediately and stay in "Pending Validation or "Pending Verification" for some time. So let's suppose, with the following simple example, that in 2 days you crunched 20 tasks (10 each day). On your first day you got credit (valid) for only 5 tasks (because the other 5 tasks are in Pending Validation status). The second day you get credited for 15 tasks, 10 for that day plus the 5 that have been validated from the previous day.
That can be answered more accurately by the techs. It used to be that the validated tasks hung around for quite a while (weeks or months), but since then the process has been optimized and is a lot faster. Generally within a few days the valid tasks are cleaned up. I really don't pay any attention to how long the valid tasks hang around.

Hope I helped you with your questions...

CJSL

Crunching like there's no tomorrow...

Validated tasks [except for beta], generally reside not longer than 24 hours on the RS pages, i.e. good chance that what you see on there does not match other My Contribution page information.

I'm using the API to extract results every 12 hours [scheduled to run just after 12:06 and 00:06 stats cutoff times] and often find that one or the other validated result has left before running the query... particularly those that were already slow in validating [holding up a batch completion].

Edit: Why that is done [speed of Scheduler, the heart of the BOINC system, and storage space... 1.2-1.3 million results per day]. The more there is, the slower the system becomes.