This is a genuine question - and I can only speak for the UK Playstation store as I don't know the prices in the US store...
But for 3 month sub it's £29.99
For 1 month sub it's £9.99
3 x £9.99 = £29.97
You save 2p by ignoring the 3 month sub... is this an oversight by Zenimax? Shouldn't it realistically be the 3 month version being priced as £27.99 to fall in line with the saving the 6 month and 12 month versions woudl subsequently give?