How does plyr merge two columns of different data.frames with same names but different values

While merging 3 data.frames using p开发者_运维知识库lyr library, I encounter some values with the same name but with different values each in different data.frames.

How does the do.call(rbind.fill,list) treat this problem: by arithmetic or geometric average?

From the help page for rbind.fill:

Combine data.frames by row, filling in missing columns. rbinds a list of data frames 
filling missing columns with NA.

So I'd expect it to fill columns that do not match with NA. It is also not necessary to use do.call() here.

dat1 <- data.frame(a = 1:2, b = 4:5)
dat2 <- data.frame(b = 3:2, c = 8:9)
dat3 <- data.frame(a = 5:6, c = 1:2)

rbind.fill(dat1, dat2, dat3)
   a  b  c
1  1  4 NA
2  2  5 NA
3 NA  3  8
4 NA  2  9
5  5 NA  1
6  6 NA  2

Are you expecting something different?