tex using the wrong macro

tex using the wrong macro

Out of curiosity I tried to weave the bibtex.web source code and compile
the resulting bibtex.tex with tex. I got some errors which led me to
investigate; I'm sure my confusion stems from incorrect assumptions on my
part.
The errors were of the missing $ kind, but point to the use of a wrong
macro. Weaved tex-files use the webmac.sty style file. In it are macros
such as \A, \As, \E , \ET and \ETs. The \E macro should be used in a math
environment.
An example of an error is:
! Missing $ inserted.
<inserted text>
$
<to be read again>
\cdot
\E->\cdot
10^
<argument> 39, 58, 60\E
Ts61
\note ...ndent 2em\baselineskip 10pt\eightrm #1~#2
.\par }
l.1135 \As39, 58, 60\ETs61.
What this shows is that the \ETs macro is only being read as far as the \E
part. Why does this happen, and why doesn't it happen with similarly named
and used \As macros?
Edit: I should add that the behaviour remains the same when adding spaces
after the offending macro.