git diff timing oddity

git version 1.7.1, Mac OS X 10.6.3, largish repo (42K files, 35K commits, well packed ~ 1GB). Difference between origin and HEAD is a single commit.

$ time git diff --stat origin  >/dev/null

real 0m0.942s user 0m0.713s sys 0m0.223s

$ time git diff --stat origin  >/dev/null

real 0m0.944s user 0m0.712s sys 0m0.227s

$ time git diff --stat origin  >/dev/null

real 0m0.944s user 0m0.712s sys 0m0.225s

$ time git diff --stat origin HEAD >/dev/null

real 0m0.022s user 0m0.006s sys 0m0.012s

$ time git diff --stat origin HEAD >/dev/null

real 0m0.022s user 0m0.006s sys 0m0.013s

$ time git diff --stat origin HEAD >/dev/null

real 0m0.023s user 0m0.006s sys 0m0.013s

These should be identical commands, so why is the "origin HEAD" form running an order of magnitude faster?

j.