And was there any explanation as to why the X and Y pulse spacings are different?
A lot of these tech sources tell you something, which can then be tested in an exam. But they fail to teach anything of practical use.
I recently researched this X and Y channel thing for air-to-air TACAN use. The client's aeroplane would give satisfactory information on Y-channels, but not on X-channels at closer ranges.
It turned out that the manufacturers of the TACAN had come up with a powerful Tx and sensitive Rx. Such was the pi$$ poor AGC response of their receiver that it couldn't tell when one X-channel pulse ended and the next began, so couldn't provide range. With AGC off it was even worse as it was simply overloaded.....
But the wider spaced Y pulses were acceptable to their system.
So, sometimes it is useful to know this $hit, but I agree that the simple task of merely driving punters around in airliners does not require such knowledge. All you really need to know is that by having both X- and Y-channel DMEs, the frequency spectrum available for DME use can be used more efficiently - hence making more DME stations available. Which is very handy for those who rely on non-GPS DME-updated FMGC systems.