AFAIK, none of the RTL type dongles have an actual 50 ohm antenna port. Remember, these were originally designed as TV tuners using RG6 or equivalent. It is a 50 ohm SMA jack connected to a possibly 50 ohm LNA interface, with output going into a 75 ohm tuner. There are possibly two thing in play here with the increased SNR:
moving receiver slightly farther away from the antenna, electrically, may reduce the amount of spurious signals being received by the antenna and pumped right back into the front end of the RTL
Impedance changed for the better
I have seen the same behavior when connecting a RTL directly to the feedpoint of my homebrew helical, as opposed to putting in a 10" jumper. SNR is much better with the jumper in. AND< I noticed the farther away I place the dongle, off axis of the antenna pattern, the better the SNR.