Well, two things in relation to that:

1. If your oven isn't really well and good pre-heated, repeated opening and closing of the oven is going to let heat escape and prolong cooking time. Don't do it. :)

2. Yeah, using cherry instead of grape may have been a factor. Cherry tomatoes are slightly larger and tend to have more seeds/liquid in them. That ratio difference (surface area to water volume) may definitely have messed up the cooking, too.