From the lil reference manual:
If join is applied to non-table arguments, it produces a list by pairing adjacent elements from x and y. If either argument is a number, it is considered "range" of that argument, and otherwise it is interpreted as a list. For example, "ABC" join 3 gives (("A",0),("B",1),("C",2)).
As you can see, it has a preference for preserving the length of the left argument if they don't match. Some functional languages call this operation on lists "zip()". Applying "join" to tables performs a natural join.
If you simply want to concatenate lists and atoms, use the comma operator (,):
a1,a2
# ("aa","bb","cc","dd","x")
(Lil doesn't have list literals per se, just atom literals and the comma operator.)
Does that make sense?