What step of fermentation is your yeast in? If you just pitch and you are going nuts then you should have a fair amount of circulation in the beer. I would bet that both the therm well and the outside glass temp are fairly similar.
If you have made it past the first few days or are conditioning then I would bet you see more variety. The thermal mass of the beer is pretty significant and the outside layers of your beer have probably dropped but your therm well is reading rig in the middle which would be the last to change.
The idea is similar to the jamil style wort chiller. Circulation causes quick heat transfer.
The temperature difference I showed in the picture has happened at all parts of the fermentation. Any time the freezer turns on to cool the beer, there is a lag in the time it takes to drop the beer temperature (thermowell probe) versus the carboy temp (external probe). That lag time leads to a temperature difference between the probes (the carboy gets colder than the liquid inside it).
The freezer definitely cycles more when the fermentation is at or near its peak, but the temperature lag and resulting differential is the same throughout.