generally, a syllable ending in a vowel (co- ) has a long sound (so, that's right, doubling the "l" would make it "collie,") and a syllable ending in a consonant (broc- ) has a short sound.
Now, when you have rob-in then by those rules you break the syllable after the b so that you can say "that's why it's a short vowel sound," and it's not as common as robot or rabbit (double letter - short vowel, single letter- long vowel).