I guess that the . is greedily parsed as part of a number, if possible, making it the float 1., instead of being part of the method call.
Spaces are not allowed around the decimal point, but you can have spaces before and after the . in a method call. If the number is followed by a space, the parse of the number is terminated, so it’s unambiguous.
Let’s look at the different cases and how they are parsed:
>>> 1.real # parsed as (1.)real -> missing '.'
>>> 1 .real # parsed as (1).real -> okay
>>> 1. real # parsed as (1.)real -> missing '.'
>>> 1 . real # parsed as (1).real -> okay
>>> 1..real # parsed as (1.).real -> okay
>>> 1 ..real # parsed as (1)..real -> one '.' too much
>>> 1.. real # parsed as (1.).real -> okay
>>> 1 .. real # parsed as (1)..real -> one '.' too much