The boil appears harder because it is. The container reduces the boil surface area, but the surface area that is boiling will boil harder and evaporation will increase in that surface area.
Evaporation provides a cooling effect (think about metho or petrol on your skin).
Even if the overall evaporation rate remains the same, the reduction of DMS will increase because of the turbulence introduced in a rolling boil provides mixing so that it's not just the top that has the concentration of DMS reduced.
I probably haven't phrased that very well, and I did just pull that explanation out of my arse (someone is welcome to correct me) but it's been a long week and I can't be stuffed articulating myself better.
