Method completion or suggestions is different for two different Strings or FixNums

Given this code:
test = "this is a string"
test2 = test.capitalize

puts test.class
puts test2.class

It outputs:
String
String

However, if I type:
test.
I get these methods popping up:
class
capitalize
ascii_only?
bytes()
bitesize
biteslice()
capitalize!
...


versus if I type:
test2.
I get these methods popping up:
class
__id__
__send__(*args)
equal?(other)
instance_eval()
instance_exec(*args)
method_missing(symbol, *args)
...

The second list does not have the string methods like capitalize and capitalize!

same behavior for 
b = 2
c = b.next
b. gives FixNum methods c. does not