You're rounding down rather than rounding to nearest.

This is 0.5.4.  I'm guessing this hasn't changed in 0.5.5:

```
159073491 iterations (about 1x10^8 hashes) in 95308 seconds. Avg rate 1669 hashes/second
```

I'd suggest you round to the nearest, rather than rounding down.  If you're only going to use one digit, that should be 2x10^8.  Maybe even use more digits.