The watermeons at the grocery store are most likely hybrids, which are a cross between two different varieties. When you plant a seed from a (non-sterile) hybrid, the resulting fruit may not come out exactly like the original fruit (may not "come true"). They might be like one parent plant, or something totally different. There are white-fleshed watermelons; perhaps a white-fleshed watermelon was an ancestor to the watermelon that you saved the seed from?
The watermeons at the grocery store are most likely hybrids, which are a cross between two different varieties. When you plant a seed from a (non-sterile) hybrid, the resulting fruit may not come out exactly like the original fruit (may not "come true"). They might be like one parent plant, or something totally different. There are white-fleshed watermelons; perhaps a white-fleshed watermelon was an ancestor to the watermelon that you saved the seed from?