Definitely.
There are different data sources. CNN has its own system of metrics. MSNBC has its own. And there are all kinds of variables in rate of reporting, AND the rate the reporting is disseminated in the media.
I've been tracking this stuff unofficially for awhile now, and I've noticed the graphics aren't always regularly available so you can make note of changes at the same time every day, for example.
And frankly, your humble tracker here sometimes has internal variables as far as not being up and "on duty" at the same times every day, and not monitoring continuously to note every change in real time. Sometimes I grab photos of the graphics when they're on delay on our TV because we rolled back on the continuous record mode (or whatever you call it) so I try to get the time check that's ON the graphic itself, rather than depending on exactly when I actually took the photo.
I'm assuming I can get at least some consistent comparisons by relying on a single source for the data. Just happens to be what MSNBC is running because that's what I wind up monitoring a majority of the time.
Long windy way of saying this is not exactly a precision-scientific polling effort. Really more aimed at watching the numbers and the trending within timeframes.