As others have mentioned, LNB drift is a drift in the frequency oscillator. Newer dish receivers can correct for a certain amount of LNB drift in software. At one point, dish would replace any LNB that drifted by 8 or more because that was about the limit that the receivers could correct for reliably.
A drift of 13 is too much...you might start to see certain transponders dropping out, especially when the weather gets warmer (LNB drift is affected by temperature). A few years ago I had a drifting LNB. Everything would be fine in the AM, but every afternoon (it was summer at the time) as the temperature increased the LNB drift would increase and channels would start dropping out. Later at night when it cooled off, the channels would come back on.
I think that you should call dish and tell them that your drift is 13. Even though it looks like your receivers can compensate for this, you are at the extreme limit for drift and they should give you a new LNB.