Where 2.6.25 came from

You said:
From my readings of the graph
the last 2 kernels have very steep merge. 2.6.19 started merging late
2.6.22 has had more late merges than its successors
2.6.20 seems to have stabilized early

Don't believe any statistics you haven't faked yourself.

There is no strong relation between the number of changes and the number of lines changed.

And none of them has any strong relation to when a kernel stabilizes.

An increase in the "count of lines changed over time" graph can e.g. be one or more of the
following:
* many bugfixes
* defconfig updates
* addition or removal of drivers

It's a nice graph, but when you ignore the fact that it contains zero information *why* lines
changed all conclusions you draw are invalid.