Extending recording time inconsistent

When I am setting a recording time for a show, I always try to add extra time. The past couple days I've noticed that the options are inconsistent. I'll try and record a show and I'm able to add 3 min, 5 min or 30 min to the end time.

Other times I'll try and record and my options for extending time is 30 min, 1hr, 1.5hrs.

Why is this? I don't want to add 30 minutes to a half hour show. Thanks