This is a common software issue - the difference between “token concatenation” and “expression evaluation” can be confusing.
Your first version simply pasted the literal content of the PROBE_X1 and _X2 named values together, and gave you the sequence
“7””3.5””4””/””2” = 73.54/2
Using Number() changed the intent from concatenation (aka text pasting…) to evaluating, giving
(7 + 3.5 + 4) / 2
As for documentation for gsender and –-grbl’s-- grblHAL’s macro usage, I have not found a good comprehensive source - let us know how it goes!
(edit - should have been more precise, grbl != grblHAL != g-code)