Each of these days represents a different difficulty level:
670 Mh/s =
1.39 BTC (6/6/11 12:30 AM)
1.07 BTC (6/7/11 9:00 AM)
0.691 BTC (6/15/11 9:00 AM)
0.438 BTC (6/24/11 7:00 AM)
So 670 Mh/s brought in 1.39 BTC a day at the beginning of June, but currently brings in .438/day.
This is NOT EVEN ONE FULL MONTH.
See why you can't trust those "monthly income calculators"?
Because choosing 9 hours before a difficulty change as your starting BTC value really makes sense as a timeline?
The start of the 1.39 BTC cycle was 5/26, so that was more or less a month. That aside though, this was a big and painful growth period for miners to be sure. Pity the foos who were buying rigs in the middle.
With the steadying of difficulty however it's possible if they stick it out they'll recoup though. Keep those fingers crossed lil buddies.