Version is a string; you can't do numeric comparisons with it, unless you cast it as a [single] first. And even that will fail if the version is ever something like 6.2.2, since that's not actually a number.
The casting that is being referred to would be something like this:
[version]$os.Version -gt [version]"6.2"
Powershell will do type conversions for you in some cases, however, "strong-typed" code will not leave things to chance. You want to do comparisons to the same type string to string, int to int, etc. and in this case version to version. Casting attempts to convert the value to the type so that a proper comparison can be done. Even when I was playing with the version cast, it didn't like X.0:
PS C:\Users\Rob> [version]10.0
Cannot convert value "10" to type "System.Version". Error: "Version string portion was too short or too long."
At line:1 char:1
+ CategoryInfo : InvalidArgument: (:) , RuntimeException
+ FullyQualifiedErrorId : InvalidCastParseTargetInvocation
PS C:\Users\Rob> [version]"10.0"
Major Minor Build Revision
----- ----- ----- --------
10 0 -1 -1
It's especially important when doing comparisons like -gt, because you are basically doing something like TEN -gt 6.2.