It's a pain isn't it!
Not sure if there is a pattern. But I do know that if a race goes off 5 mins early then they will all go 5 mins early at that track for the rest of the evening. They often go off late as well, same rule applies - once one goes late, they all go late. I have come to the conclusion that Americans just can't tell the time.
My general rule is that I watch the off time at each track before trading on any of them. As you say it can work out expensive otherwise.