If I write the following snippet:

${3:x}${1:$TM_SELECTED_TEXT}$3$2

activate it, then press tab twice so the first 'x' is selected, I would expect what I type to be mirrored in the other 'x' too. Why isn't it?

Similarly,

${1:x}${2:y}$1$2

doesn't do what I'd expect: the 'x' is mirrored, but the 'y' is not.

What am I missing?

Thanks,
Robin