Dating, to be successful long term, requires being able to trust the other person.
One of the ways trust is built is when a person does what they say they are going to do. This allows you to begin to count on someone.
I think there are two ways to look at this:
We humans are very inconsistent and incongruent people. We say we want to lose weight and then eat like crazy, etc. So some disconnect between words and actions is to be expected from fallible human beings.
At the same time, one of the ways we learn to trust someone is with a track record that says this person does what they say they are going to do, with very few exceptions.
What you need to look for are patterns over time. Does this person say one thing and do another? Make promises and then break them? Always place the blame for not coming through on someone or something else?
These are all warning signs of a problem.
Here is the bottom line – if you have to choose between believing the words and believing the behavior, go with the behavior.