Reporting on historical value changes - Results inconsistent

Hi all

I'm trying to see how many leads moved from Cold to Warm in Q1 and Q2 this year.

I did some interim reporting in May for Q2 and had 311 leads move from Cold to Warm . 

The same smart list for all of Q2 is now giving me 102 leads .

When changing the smart list to Q1, the result is 0. I was expecting around 103 from previous reporting.

Am I doing something wrong with my smart list, or am I misunderstanding how data is captured/stored, which is impacting my results?

Thanks