The mathematician misses three feet to the left and the physicist misses three feet to the right. If you take the average of those two, like in statistics, it's right in the middle and would hit the deer.

In simplest terms, statisticians are stereotypically known for wanting to simplify curves. So, if you see a plot graph of data and notice a cloud of points that appear to angle in a certain direction, statisticians will sometimes draw a line from the first average point to the last average point and say that line represents the data. So the joke is that statisticians throw out the individual data and say the average represents the real world, so three left and three right equals and average of a direct hit.

For the physicist, they're known for representing everything as spheres to simplify their math and create approximate calculations. This means that back-of-the-envelope calculations will get you close but won't be precise.

Mathematicians are said to only calculate for one variable at a time, in this case, trajectory without accounting for wind or air resistance. Mathematicians also often joke about using "dots in space."

The overall joke is that these are three incredibly intelligent people abusing the most boiled down versions of their fields and failing terribly because they refuse to experience the world in anything other than numbers and figures.

I hope you are now able to enjoy the joke more, although I imagine it's now more tedious than enjoyable.