To achieve 16 channels MPX must use non-standard timing Max, as sixteen periods of two-milliseconds plus a few milliseconds synch simply wont fit in a standard 20ms PPM frame.
I dont know how they do it, they could send alternate frames of channels 1-8 and 9-16, or maybe they use short channel timing - or it could be a 'standard' channel timing in a 40ms frame.
The tester would do one of three things:
Alternate frames - each channel would rapidly flicker between 1/9, 2/10, 3/11 etc and would be unreadable
Short channel timing - it woudlnt recognise the signal without timing changes to the code
Standard channel timing in a 40ms frame would read all channels ok and would display ch1 to ch6
English is wonderfully strange Dave, in fact the OED supports both spellings but the preferred spelling of any past particlple ending in "...ized" such as 'synthesized' or 'analyzed' is with the Z. Yet 'analyser' and 'synthesiser' have the preferred S.