Two Theories

All new sites Submitted to Google are now placed in a theoretical “Sandbox” away from the real world really. This period of time used to determine Spammy sites from the ones with unique content. Thus preventing Spammy sites getting good SERPS.

The second idea is that this actually has absolutely nothing to do with the physical site, rather the backlinks are only given partial credit for a probation period. Preventing sites shooting up the SERPS.

Conclusion is that Google are making an attempt to filter out spammy websites and are also as an affect preventing new websites establishing well in there SERPS for a certain amount of time. This may even be linked to a level of backlinks reached. Any thoughts/comments would be good!