subbottom of memoir puts margin around minipages

I have many figures in my document that consists of some minipages which take up the full textwidth. Now I wanted two of these figures to appear combined as subfigures. Since the document is in memoir, I decided to give subbottom a try. Unfortunately, subbottom in combination with minipage sets a margin around the figure, and I do not know how to get rid of it. That means, the figure cannot have the full textwidth. Here is an MWE:

documentclass[11pt]{memoir}
usepackage[english]{babel}
newsubfloat{figure}
pagestyle{ruled}
begin{document}

begin{figure}
subbottom{
begin{minipage}{.495textwidth}
a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
end{minipage}
begin{minipage}{.495textwidth}
b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b
end{minipage}
}
end{figure}

a
end{document}

As can be seen in relation to the rule, the figure is a little bit shifted to the right.
If I omit the subbottom, everything is fine. And if I omit the minipage, the text inside the subbottom also starts at the left end. But in combination it does not work.