You may occasionally see a 'book it' time slot that has a fixed or frozen duration / start time. When you see that, you are looking at a designated start time for that court on that day. This has been implemented by your tennis center to prevent little 30 or 60 minute gaps of (unuseable) court time by scheduling play to start back-to-back.
If you require less time, or need to start your reservation at a different time, please check other courts for different booking rules.
Here is an example: (4PM time slot so that two 90 minute matches can be played before evening activities start at 7PM)