OTBI- Filter is based on results of another analysis

Content

I am writing a report 1 that take all of our companies invoices and shrink them down to a population of only specific invoices.

Then in report 2 I want to be based on report 1's results and eventually apply more logic based on session user.

When i build report 1 i get say 1500 results. When i built report 2 without any filters other than basing it on report 1 i get a different number? 

If I base report 2 on report 1 and do so on a given field doesnt that mean i am only grabbing results based on that field and then using those? I think I am a bit rusty or confused on this.