"Last (n) " variables in SPM reports

Hi:

We are at SPM 2.1 SP6.

we have data for 2008-9-10-11 (up to a certain point in 2011).

When I create a SPM report with Last (1) year, It shows me data for Year 2009.

When I create a report with Last (12) month, it shows me some months in 2008, some in 2009, some in 2010 and one in 2011.

So - few questions:

1. Any time variable with Last (n), do they use date of execution to calculate the starting point? Or "Last data upload date" - or any other logic?

2. How come Last (1) year goes back to 2008? Or random months in when chosen Last (12) months?

Any help is appreciated...

Thanks