Border too thick -- Photoshop 6

Sometimes when I go to add a border around my icon, it turns out too thick and looks like it's about 2 pixels thick instead of the 1 pixel I specified. I do my border by selecting the icon, Select --> Modify --> Border and choosing 1 pixel. Then I go to Edit --> Fill. Sometimes it will turn out good like the first icon below but other times, it turns out too thick like the second icon. Does anyone have any ideas why this might be?