Stuck mining.thegenesisblock.com Difficulty Increase / Month (%) 76-77?

[eurgbp2011]

Network Stats
Current Difficulty (MM)   86.93
Difficulty Increase / Month 76-77 (%)

For several months DI stuck in 76-77 range why so?
when historical data shows
Historical Difficulty Increase
30 Day   132 %
60 Day   232 %
90 Day   457 %

Is it a bug in their script?

[tgb_jeff]

mining.thegenesisblock.com

We use the average monthly increase over the last 90 days. So a 77% increase per 30 day period compounded over 90 days (3 periods) gives a 457% increase. This was done to smooth out the spiking that occurs when the 30 day window changes between 2 and 3 difficulty periods, and to reduce the impact if there is a large spike in network speed in the near term that is not typical of the historical growth trend.