Your monthly interest rate isn't quite right, it's 7% / 12, rather than 1.07^(1/12). i.e., if the annual interest rate is 100% (doubles every year), then it'd

Your monthly interest rate isn't quite right, it's 7% / 12, rather than 1.07^(1/12). i.e., if the annual interest rate is 100% (doubles every year), then it'd be incorrect to say the 6-month interest is half that (50%). As two terms of 50% would be a 125% increase, not 100%. To get to 100% you'd need two terms of 41%. If you apply this to the monthly rate it drops by a little. Compounded over all those years, the total amount drops to by about 6%. As the one value drops by a larger amount in absolute terms, the earnings difference actually drops by about $50k. Either way, it's still a ridiculous difference and your point fully stands.