Well, for the same reasons you might want to track your performance in a certain situation, it might be a good insight to track down the interruptions you're facing while using the Pomodoro technique. Even if you are an experienced user.
For this technique to work well, the less you interrupt the 25min or so minutes, the better you will perform in those bursts. Any interrupts, you might be better doing at the 5min pause, if capable. Or even better in a big pause after 4 or 5 cicles.
The difference, though, of the internal and the external interruption is that the first is related to how you work and if you can focus on that particular activity during the burst, like not checking email, facebook or twitter during it. And that's what you can train to get better of. But the late one, might be more related to the environment you work. If it's too noisy or people call you all the time, than you could need another approach to your task, such as changing rooms, notifying people you are busy or just wearing headphone (in my case, that help a lot because people know that it's an extra effort to reach me).
So, the basic thing is being aware of how much you are being interrupted in the middle of the Pomodoro burst. And that might happen even if you are used to it, because sometimes is just beyond your control. And that could help you to see what could be causing that and ways so you can reduce or avoid that.
Doing some more research, I also found that:
The real objective, taken from Pomodoro Technique official book (pdf):
The first objective to achieve in cutting down on interruptions is to be aware of the number and type of internal interruptions. Observe them, accept them, and schedule them or delete them, as the case may be.
The second objective to achieve in order to cut down on interruptions is to be aware of
the number and type of external interruptions. Negotiate them and reschedule them depending
on the real degree of urgency.
So, it's clearly for optimizing the quality of the Pomodoro cicles, as said: "(...) you can also include the total number of internal and external interruptions on the Records Sheet to observe them and try to minimize them over time."