One possibility that could explain the Nest coming on too often is that presence sensing may be turned on (or Home/Away Assist, depending on your model). For troubleshooting you want to turn off that "smart" feature. Then your Nest ought to run strictly according to your schedule.
What schedule do you have set up in your Nest? Is it something like:
7am 18
8am 13
6pm 18
8pm 13
That's an example of a schedule that would run the Nest mostly just for an hour in the morning and two in the evening -- but yes, if the room temperature drops below 13 the heat will come on at other times.
