Tank readings are the most frustrating thing for any RV owner. RVs are cheap even when they cost a million dollars. The cheap little button sensors they install in the tanks get gunked up quickly and often there will be enough gunk on the side of the tank to allow current flow between the sensors giving you false readings. Another complaint you hear often is, "How can my fresh tank go from Full to 2/3 when all I did was wash my hands?"
There are 4 sensors in each tank. Common, 1/3, 2/3 and Full. Current runs between the sensors. You can have a full tank and run just enough water to lower the fluid level to no longer reach the Full sensor and it will indicate 2/3rd full because that's the highest sensor still getting current even though the tank is still "almost" full. A little below 2/3 indicates 1/3 and a little below 1/3 indicates empty. Same thing in reverse as you fill your tanks.
Obviously a better design would be a float or a strip sensor with a continuous gauge like we have in our cars but it is what it is. Everybody complains about them, doesn't trust them, uses a broom handle the check the black tank, uses a flashlight to check the fresh tanks, goes to all kinds of trouble and spends money on goofy gadgets to rinse their tanks, etc. It seems like half the RV internet is dedicated to tank cleaning tips, tricks, techniques and gadgets.
In your case I suspect the side of your tank was wet from the running water and it allowed current to flow to the 2/3 sensor. If you left it alone and checked an hour later it might have dried out and been back to normal. One tip that worked for me is to upgrade the button sensors in your tanks to Horst Miracle Probes. I found out about them on this forum because somebody lists them in their signature as an upgrade they've done to their x23B. I put mine in about a year and a half ago and my readings have been spot on since. (no, I don't work for or sell the things) Best upgrade I've done to my trailer. I take that back. The electric jack I put on is the best. :-)
Horst Mircacle Probes - Get accurate reading on your RV's holding tanks