which frequencies are all?

I was approaching class C airspace for landing at the primary airport
for which the class C airspace is designated and I listened to the
ATIS. The ATIS said "all frequencies are combined on
TowerFrequency". So I tuned to TowerFrequency and made my radio
call to approach. The response that I got was that I needed to call
approach on the approach frequency. So, I guess that begs the
question ... when the ATIS says "all frequencies are combined", which
frequencies are included in "all frequencies"?

Thank-you in advance.