Users / Clients are booking times that shouldn't be available

We are having issues with users booking times that shouldn't be available. For example we have multiple times available

• 9:30am
• 10:50am
• 11:30am
• 2:10pm
• 2:50pm
• 3:30pm

But occasionally users are booking times at other times (eg. 2:20pm) - we originally thought this was caused because we allowed the users to set their own timezone (therefore times such as 3:00am were available) - but we've disabled that. Anyone else have this issue? Any idea on how to prevent / correct it?

These are the correct times

Here's an example of a user taking a screenshot of a time available that shouldn't be. 
PS - all our hours are the same for everyday / each week