When more than one context length is given, GNU diff uses the biggest one, and a plain -u or -c counts as 3. We just keep the last number that was given, and a plain -u or -c doesn't count at all. So -U1 -u gives 1 line of context here and 3 with GNU.

$ printf 'a\nb\nc\nd\ne\nf\ng\nh\n' > a
$ printf 'a\nB\nc\nd\ne\nf\nG\nh\n' > b
$ diff -U1 -u a b | sed -n 3p        # GNU diff 3.10
@@ -1,8 +1,8 @@
$ diffutils diff -U1 -u a b | sed -n 3p
@@ -1,3 +1,3 @@

Same thing with -U2 -U1 (GNU uses 2, we use 1) and with -C1 -c.