I've constructed an Electoral Calculator in excel using both national and local polls - and the interesting thing is that even though the GOP are well ahead in the polls (latest RCP Average - 5.8%) they're still not in safe majority territory. At a lead of 5.8%, my calculator reckons the result would be :
GOP 222 DEM 213
If you exclude the local polls and apply the swing (8.3%) to all 435 districts the result is practically the same :
GOP 223 DEM 212
If the two parties were to tie nationally, the result would be as follows :
GOP 197 DEM 238
I wonder is perhaps turnout lower in massively Democrat Congressional district areas, i.e inner city areas, which might explain the discrepancy....