I guess it shows they are basing it of of how many BTC they think it will mine, seems FIAT changes and does make them more cash. Surprising move in my mind.
bitmain are a real business and they have their numbers worked out pretty well (or have some good brains behind their money), so if you want to make any analysis on their possible move(s) it better be based on empirical evidence (not an un-informed hunch) or sound business logic (even both plus many others!). Unlike some bitcoin chip fabricating ventures, they seem to have got some kind of balance and (presumably) use the home mining market as an outlet for the gear they need to decomission (or as a testbed) as clearly it is more profitable by magnitudes to self mine.
Anyway, your memory probably let you down on this one as bitmain have previously raised the price (of the S5 no other ...) more than once in a single day.