Alex_TJ - 5:48 pm on Nov 24, 2012 (gmt 0)
This doesn't explain the Mac, but I've noticed that Chrome seems to be used to test out new rankings. When I was recovering from Panda I often saw a large influx of Chrome visitors at a new higher ranking for a few hours (quite easy to spot when you had 200 uniques...) and then they went away, before that new ranking appeared a few days later for everybody. It was almost as if chrome was an advance party to test out the new position before rolling it out.
That might mean good news for you - how did your rankings work out?