I just searched the rules for "1237" and didn't find it. Rule 40d says "majority of votes entitled to be cast". So if Rubio's bound votes aren't entitled to be cast on the first ballot, because he dropped out and/or because rule 40b still applies and he didn't qualify, then that lowers the majority number by (272/2=136) that Trump has to hit. Thus Trump needs 1101. But then there were other delegates that dropped out too.
Enjoy!!!!
Rubio has 173 delegates.
Carson has 9 delegates.
Bush has 4 delegates.
Rand has 1 delegate.
Mike Dale has 1 delegate.
Carleton Sneed has 1 delegate.
11 delegates are uncommitted.
Kasich has 144 delegates.
Cruz has 533 delegates.
Trump has 757 delegates.
Per this tracking site: http://www.thegreenpapers.com/P16/R
Perhaps, since he is assuming Kasich won’t be elegible, he is subtracting all the non-Trump delegates.