The difference is due to the differences in interest payments received each year.
The principal payments at maturity will be the same for both bonds.
Using the calculator, the yield to maturity of Bond A is 11.77% and the yield to maturity of Bond B is 11.59% with the 10% reinvestment rate for the interest payments.
Mark would be better off investing in Bond A.
The reasoning behind this result is that for both bonds the principal is priced to yield the YTM of 12%.
However, Bond B is more dependent upon the reinvestment of the large coupon payment at the YTM to earn the 12% than is the lower coupon payment of Bond A.