From my experience it seems like what they tell you in advance and what they actually do are often two different things at jazz festivals. You are often at the mercy of the whims of the stage manager- sometimes they will cut some slack for one artist while cutting another off short (yes, it can get very "political"). It might have had something to do with efforts to stay on schedule, or possibly another group needing some extra set-up time. No question it's a raw deal, but I think in anticipation of this one might plan a shorter program with the option to "audible" an extra tune or two if time permits. It means you might not get to end with the desired tune, but at least it will get played. You just never know how it's all going to go down.
We had the opposite problem twice this year. While opening for Macy Gray they told us as we were going on stage that we had to do 90 minutes instead of 45.
Recently, we were told we had 20 more minutes to go, by one person and that we were perfectly on time to do "the encore" by another. While opening for the Pointer Sisters.. a few months back...we got our set cut by 10 minutes because of an approaching storm. You never know.
Our Detroit Jazz Festival appearance was right on time...but honestly, we could have gone longer. Ground transportation kept losing musicians.