Why does the transition duration vary so much

Sorry, not a very good way of putting my problem. According to the tutorials and instructions, transition times can be adjusted by either double clicking the transition to open the inspector or clicking the cogwheel on the transition. However when I put in the time I want it very often doesn't apply it. E.g. I chsnge the time from 2.4s to say 6s for a long disolve but when I look at the transition time it is still 2.4s or sometimes an arbitrary number. For the project I'm on I need a long final disolve of about 7s. I've put this into the Inspector about a dozen times and it always comes out as 1.9s. I do not use overlap transition just the ordinairy one. There seems to be no sense in this and it makes defining the length of the project and synchronising music a real pain. Anyone else got this problem?

Wilf