xts operations yield wrong result - r

Question

Assume I have three xts objects a, m, s, indexed with the same time slots, I want to compute abs((a*20)-m)/s. This works in the following simple case:
bla <- data.frame(c("2016-09-03 13:00", "2016-09-03 13:10", "2016-09-03 13:20"),c(1,2,3), c(4,5,6), c(7,8,9))
names(bla) <- c('ts','lin','qua','cub')
a <- as.xts(x = bla[,c('lin','qua','cub')], order.by=as.POSIXct(bla$ts)
... similar for m and s...
abs((a*20)-m)/s
gives the correct results.
When I go to my real data, I see different behaviour:
> class(a)
[1] "xts" "zoo"
> class(m)
[1] "xts" "zoo"
> class(s)
[1] "xts" "zoo"
> dim(a)
[1] 1 4650
> dim(m)
[1] 1 4650
> dim(s)
[1] 1 4650
Also the column names are the same:
> setdiff(names(a),names(m))
character(0)
> setdiff(names(m),names(s))
character(0)
Now when I do n <- abs((a*20)-m)/s I get
> n[1,feature]
feature
2016-09-08 14:00:00 12687075516
but if I do the computation by hand:
> aa <- coredata((a*20)[1,feature])[1,1]
> mm <- coredata(m[1,feature])[1,1]
> ss <- coredata(s[1,feature])[1,1]
> abs(aa-mm)/ss
feature
0.0005893713
Just to give the original values:
> a[1,feature]
feature
2016-09-08 14:00:00 27955015680
> m[1,feature]
feature
2016-09-08 14:00:00 559150430034
> s[1,feature]
feature
2016-09-08 14:00:00 85033719103
Can anyone explain this discrepancy?
Thanks a lot
Norbert

Answer 1

Self answering: the error was that I believed that xts is more intelligent in the sense that a/b considers column names, which it does not.
> a
lin qua cub
2016-09-03 13:00:00 1 4 7
2016-09-03 13:10:00 2 5 8
2016-09-03 13:20:00 3 6 9
> b
qua lin cub
2016-09-03 13:00:00 2 3 4
2016-09-03 13:10:00 2 3 4
2016-09-03 13:20:00 2 3 4
> a/b
lin qua cub
2016-09-03 13:00:00 0.5 1.333333 1.75
2016-09-03 13:10:00 1.0 1.666667 2.00
2016-09-03 13:20:00 1.5 2.000000 2.25
Division is done via the underlying matrix without taking care of column names. That is the reason while even if the set of column names coincide, the results are wrong.