Setting inline hook to a function identical to default in knitr turns of exponential formatting

Hello List,

while dealing with a questin of 'xiaodao' (
http://r.789695.n4.nabble.com/Problem-with-large-small-numbers-in-knitr-tp4653986.html)

I noticed that copying the default inline hook function obtained by
knit_hooks$get("inline")
into a knit_hook$set(inline = <...>) call turns off exponential
fomatting in the resulting .tex file.

I used a stripped version of 'xiaodao's example:
\documentclass{article}
\begin{document}

<<>>=
a<-1e-13
b<-2.5e-10
ssrr<-123456.12
ssru<-123400.00
@

$
c=\Sexpr{a}/\Sexpr{b}
f=\Sexpr{ssrr-ssru}/\Sexpr{ssru}
$

\end{document}

so:
knit_hooks$restore()
knit_hooks$get("inline")

## yields:
## function (x)
## {
##    if (is.numeric(x))
##        x = round(x, getOption("digits"))
##    paste(as.character(x), collapse = ", ")
## }
## <environment: namespace:knitr>

knit("FILENAME.Rnw")

## .tex-file:
## c=$10^{-13}$/$2.5\times 10^{-10}$
## f=56.12/$1.234\times 10^{5}$


## then run knit_hooks$set() with exactly the same function:
knit_hooks$set(inline= function (x)
{
        if (is.numeric(x))
                x = round(x, getOption("digits"))
        paste(as.character(x), collapse = ", ")
}
)
knit("FILENAME.Rnw")

## .tex-File
## c=0/0
## f=56.12/123400

knit_hooks$get("inline")
## function (x)
## {
##      if (is.numeric(x))
##              x = round(x, getOption("digits"))
##      paste(as.character(x), collapse = ", ")
## }


The only thing that changed is no <environment: namespace:knitr> is
printed anymore

Why does knitr change its output?

Regards, Moritz