There is more than"b" on git _identity in Arducoper-v2 .px4. how to solve this

the arducoper-v2 .px4 (copter-3.4.6) shows 801kb that I make by the code from github without any change
.but the arducoper-v2 .px4(stable-px4-heli ) shows 796kb form why not the same?
arducoper-v2 .px4 form shows: “git_identity”: “stable_manual_flight-15972-gde6b667”,
the other shows: “git_identity”: "b’stable_manual_flight-15972-gde6b667-dirty’"
what does the dirty mean?

I deleted the ardupilot,and git clone again. make px4-v2, the arducopter-v2.px4 shows:

"git_identity": "b'stable_manual_flight-15972-gde6b667'"

more than “b”. Is that right? why is different from the official website