At some point in the distant past, we started normalizing away (removing, really) about ids in parser test output. The rationale at the time was that parsing was asynchronous and we couldn't guarantee that the exact same about IDs would be generated in two different runs, so they were causing spurious test failures and a bunch of update noise in tests.
Since that time parsing has changed to be almost entirely synchronous, but then another source of variance emerged because Parsoid would parse in multiple different DOM documents and in the process of cloning fragments from one document to another the about IDs would get incremented in unpredictable ways. I believe that issue has been mostly resolved now as well.
We should revisit our handling of about ids and stop stripping them from parser test results. If they are still inconsistent between runs, we can do a renumbering pass as part of normalization, but we shouldn't remove them entirely.