irb(main):001:0> a = [12,16,5,9,11,5,4]
=> [12, 16, 5, 9, 11, 5, 4]
irb(main):002:0> a.reverse
=> [4, 5, 11, 9, 5, 16, 12]
I'm not seeing what you're seeing.
Edit: Expanding on what Ben noticed, you may be reversing a string.
irb(main):005:0> "12,16,5,9,11,5,4".reverse
=> "4,5,11,9,5,61,21"
If you have to reverse a string in that manner, you should do something like the following:
irb(main):008:0> "12,16,5,9,11,5,4".split(",").reverse.join(",")
=> "4,5,11,9,5,16,12"