Arduino Serial TX to iPhone RX voltage drop?
someone probably mucked with resistors until the voltage came right, or maybe there's a pullup/down resistor sitting there on the input.

Or it's a high impedance input and 1.7mA is the most they feel comfortable pushing through the input's protection diodes

It depends a lot on what the input circuit and impedance is - you might be better off (and safer) building a voltage divider (2 resistors of your own) you really don't want to be pulling inputs above their Vdd - you end up dumping that current into the protection diodes and making stuff hot (or mildy warm) - better off to start with a simple voltage divider and then looking at the signal levels on a 'scope - tweak to get the result near to 3.3v but lower