[[ ]] and [ ] work similar, but the implementation is different.

Code:

$ type [
[ is a shell builtin
$ type [[
[[ is a shell keyword

The [ is a builtin command. In fact it is an alias of the test command. (When run as [ the last argument must be ]. Command arguments may be redirected with < and >, and that is what caused your problem. The redirection can be prevented by quoting "<" ">" or \< \>, but the standard operators are -lt and -gt. As with all command arguments, you should use "quotes" to prevent word splitting and globbing.
The [[ ]] is a compound, built into the shell interpreter. Because it is not a command, its content can be freely designed, and in fact a < and > are valid integer comparators. And the values are not subject to globbing, and word splitting happens before variable substitution (i.e. you need to quote "two words" to be seen as one value but not $word even if it's value is "two words").