science question here about drop in proof

[jon1163]

I've noticed it in every single one of my runs regardless of what I'm running, neutral, All Grain, sugar head, Etc, that my proof starts off in the first jar just a hair lower then my early heads then I get the typical drop-in proof over the duration of the run.

Here's a typical strip run example for me:
jar#-ABV
1-64
2-65
3-63
4-62
5-60
6-60
7-59
8-55
9-50
10-48
11-45
12-40
13-35
14-30
15-22
16-15

Why is the initial jar almost always a hair lower than my early heads?