That same thing happened to my 75 Bug with an electric fuel pump. The pump was wired to run only when the ignition was on. I parked it in the driveway facing uphill for weeks at a time and it's a pretty decent slope so the gas tank was actually above the carb. My only guess was the weight of the gas, that somehow gravity fed thru the pump, overpowered the float needle and leaked slowly over time into the intake then past the rings.
It happened to me again after I replaced the needle assembly that I though might have been faulty and also checked the float level. I now park it nose downhill and haven't had any gas in the oil since.....
Mac