Sleeping on a Zebra

LLC has been sleeping on a zebra, and no, I don't mean one of her stuffed animals.  I mean her mattress.

When I was changing her sheets the other day I again noticed that her mattress, below the protective mattress cover that zips around it, has gone all stripy.  It's really strange.  The stripes are a pale yellow color and appear to have emerged in the pattern of the wooden crib base.  It's like some...chemical(?) in the mattress is reacting with the wood and causing a fade of sorts.

Our mattress is from Mamas and Papas.  I should really take a picture of it to share here but I'm not changing the sheets until the weekend so you'll have to use your imagination for now, unless of course you know what I'm talking about. 

Has this happened to anyone else and can anyone tell me why this happens?