I am assuming your layout was 12 inches at 300dpi
20 pixels is only .17cm or .067inch. Sounds to me she is giving you wrong information and they haven't set your image up correctly or it would have cut them all off the same. Your image must not have been centered. I am in australia so not costco but find generally printing places apart form the online photobook ones don't really know what they are doing lol.
What I do is allow 75pixels [1/4 inch or .64cm] on my page that doesnt have anything important on it. Just the backround but no photos or text. If the background right up to the edges is important [if it is decorated] I would go to costco get them to write down for you the size of the image they want to print on 12 inches and the bleed they need and if you make it that size then when you take it in tell them you have made it specifically to their sizing and you want it centered and if its wrong or randomly cropped tell them to do it again lol.
The bleed is just the area on the edges that may or may not print depending on their machine and how they set it up - how much they zoom in etc. It is just like giving them a little leeway if that makes sense.
I know when I make photobooks and place my photos that are 12 inches you can always zoom in or out a little and I am sure their software lets them do that as well.
Hopefully someone else has printed at costco and can help you more but if you are not happy just tell them in as nice as possible of course. i once had a whole book reprinted as it had pages randomly cut off lol.
In the meantime I am looking at their website and will see what info I can get for you.