Seedless grapes are produced by grafting. The seedless vines are attached to established vines of seeded varieties. My childhood neighbor had Thompson seedless grapes in his back yard. As the vines grew untended, the muscat vines at the base of the plants started to produce large, sweet, seeded grapes.
The grapes you buy in the store are, however, treated with growth hormones. Without the chemicals they are the size of large peas, but with the same amount of sweetness and flavor -- just less water.
, you now know the truth. So yes, indeed, you do have that 20%!