checking conflicts on build stage

[Sergey Matveychuk]

Hello.

Who did the subject and why?

# portupgrade -f -o /usr/ports/mail/exim exim-mysql-4.71
===>  exim-4.76 conflicts with installed package(s):
       exim-mysql-4.71

       They install files into the same place.
       You may want to stop build with Ctrl + C.
===>  Vulnerability check disabled, database not found
===>  License check disabled, port has not defined LICENSE
===>  Found saved configuration for exim-4.76
=> exim-4.76.tar.bz2 doesn't seem to exist in /usr/ports/distfiles/exim.
=> Attempting to fetch ftp://ftp.exim.org/pub/exim/exim4/exim-4.76.tar.bz2
exim-4.76.tar.bz2                             100% of 1568 kB   55 kBps 
00m00s

===>  exim-4.76 conflicts with installed package(s):
       exim-mysql-4.71

       They will not build together.
       Please remove them first with pkg_delete(1).

Why conflicts checked on fetch stage (but it's not fatal) and on build 
stage (fatal)? Of course, portupgrade do pkg_delete *after* build.

-- 
Sem.