The interior of a Potato is pretty much Starch and Water. So, my guess would be they have a lower ratio of Starch to Water. As for the GI as another user mentioned. Candian Potatoes tend to have a marginally lower GI than American Potatoes, even for the same variety.
Code:
Description GI CHO GL
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Potato, Russet Burbank, baked without fat (Canada) 56 21.8 12.2
Potato, Ontario, white, baked in skin (Canada) 60 20.3 12.2
Potato, Russet Burbank, baked without fat (USA)* 71 21.8 17.0
Potato, Russet Burbank, baked without fat, 45-60 min (USA) 78 21.8 17.0
Potato, Russet Burbank, baked without fat (USA) 111 21.8 24.1
Potato, boiled in brine, refrigerated, reheated (India) 23 19.5 4.5
Potato, uspecified type (Kenya, Africa) 24 19.5 4.7
Potato, white, cooked (Romania) 41 19.5 8.0
Potato, New (Canada) 47 19.5 9.2
Potato, New (Canada) 54 19.5 10.5
Potato, white, boiled (Canada) 54 19.5 10.5
Potato, Pontiac, peeled, boiled 30 min (Australia) 56 19.5 10.9
Potato, boiled (Australia) 56 19.5 10.9
Potato, New (Canada) 70 19.5 13.7
Potato, Nardine, boiled (New Zealand) 70 19.5 13.7
Potato, boiled in salted water (India) 76 19.5 14.8
Potato, new, unpeeled and boiled 20 min (Australia) 78 19.5 15.2
Potato, Pontiac, peeled and nuke for 6-7.5 (Australia) 79 19.5 15.4
Potato, microwaved (USA) 82 19.5 16.0
Potato, Sebago, peeled, boiled 35 min (Australia) 87 19.5 17.0
Potato, Pontiac, peeled, boiled 35 min (Australia) 88 19.5 17.2
Potato, Desiree, peeled, boiled 35 min (Australia) 101 19.5 19.7
*Serving Size was 200g, instead of the standard 150g...So, I used 75% to calculate for a 150g Serving Size.