“But there are only 763 delegates left to win.”
Looks like there are 839 delegates remaining in future primaries/caucuses.
North Dakota 28
Wisconsin 42
New York 95
Connecticut 28
Delaware 16
Maryland 38
Pennsylvania 71
Rhode Island 19
Indiana 57
Nebraska 36
West Virginia 34
Oregon 28
Washington 44
California 172
Montana 7-Jun 27
New Jersey 51
New Mexico 24
South Dakota 29
Totals 839
You missed Colorado and American Samoa.
I think when they talk about how many delegates remain available they don’t include unbound delegates. For example, your list has 71 available for PA, but 54 are unbound. That means they are not available for purposes of reaching 1237 before the convention, regardless of who wins the state. North Dakota has 28 unbound - same deal. This is why some calculations show 839 remaining and others show only 763.
However, I happen to think that if Trump wins a strong plurality, say 1180, but not the 1237 majority, the unbound delegates from states that Trump won would be under enormous pressure to pledge themselves to Trump on the second ballot, especially if he had won those states convincingly.
Remember, the delegates have to go home and face the voters in their state, who would be very angry with any delegates who voted differently than the state voted.
This is why even if Trump falls a bit short of the 1237 going into the convention, he may still clinch it on the second or third ballot.
Pundits like to talk about a brokered or contested convention because it would be exciting and great for ratings, but the Republican Party has plenty of reasons to avoid that chaos and turmoil, and quickly resolve it in a way that ratifies the will of the people. I know they are known as the stupid party, but can they really be THAT stupid?