use of bquote

When I use run this expression

a <- 2; b <- 3; xyplot(1:10 ~ a*(1:10), sub = c(bquote(a == .(a) ~ b==.(b))))

the subtitle contains three copies of the "a = 2  b = 3" phrase.
Why does it do that?  How do I tell it to give me only one copy?

Rich