RS-232 Zero (-7V) drops when I connect db9 connectors
Inside your converter are a USB interface chip and a level converter. The common types of level converters (MAX232 an relatives) use a switched (flying?) capacitor system to create the ~ +/10V from the +5V presented to them. It does this by first converting +5V to +10V, and then converting +10V to -10V. As a result, the -10V supply has a somewhat higher impedance than the +10V supply, so under the same load it will drop more.

Additionally, I am not sure that the load presented by an RS232 receiver is symmetric. The MAX232 datasheet shows a 5k load to ground, but there is no data on the current into the input under various input voltages.