I've put together some data which show exactly why and where the delegate counts from CBS, CNN, and NBC differ. Results after the jump.
All three networks agree on delegate counts for the following states:
That's where the agreement stops.
CBS totals for the remaining states, and totals, are as follows:
Contrast this with the CNN figures:
Factoring out the "consensus" states, note that CBS's projections add to Obama's pledged delegate advantage by 7 (586-579), while CNN's detract from this advantage by 8 (573-581).
This means CBS's estimate is 15 delegates more advantageous to Obama than CNN's.
To look at the margins state-by-state, CNN's makes an estimate for Virgin Islands, which CBS does not; this yields Obama 3 more delegates net. However, its estimates for other states yield Obama 18 fewer net delegates.
8 of this last difference of 18 arise from one state - Washington (CBS: Obama 43, Clinton 15; CNN: Obama 35, Clinton 15).
The other 11 of these 18 accumulate in ones and twos from Alabama, Georgia, Illinois, Lousiana, and Tennessee.
It appears that, for whatever reason, where their margins differ, the CBS estimate is consistently more favorable to Obama than the CNN estimate.
However, keep in mind that the differences are relatively small: 15 out of more than 1100 delegates pledged.
CNN's superdelegate estimate is 19 delegates less generous to Obama than is CBS's: CNN puts Obama's disadvantage here at 89 (135-224), where CBS pegs it at 70 (140-201).
Together, the 15-delegate difference in the pledged delegate estimates, along with the 19-delegate difference in the superdelegate estimates, make up the 34-delegate difference between the CBS and CNN overall estimate. CBS estimates a 7 delegate lead for Obama; CNN's a 27 delegate lead for Clinton.
The CBS/CNN difference is trivial, however, compared to the difference of both to the NBC estimate:
Strikingly, NBC makes gives no estimate for superdelegates. This makes the overall estimate more favorable to Obama: for NBC, Obama leads Clinton by 54 delegates.
In another way, however, the NBC estimate is much less favorable to Obama. With no estimate of the superdelegate margin, NBC's bottom-line estimate and its pledged delegate estimate are identical.
NBC's margin for Obama in pledged delegates of 54, then, is less favorable to him than CNN's (62) or CBS's (77).
The difference seems explained by NBC's leaving more delegates unallocated: NBC's total under TBD (108) is much greater than that for CNN (52) or CBS (66).
Now to post this, before my math gets outdated.