There has been a general increase in the number
of adjacent channels on UHF. I don't know if
the analog TV tuners are better, or if they somehow
improved the transmitters, but think about it
-- Cable TV has had adjacent channels since the
beginning, so there has to have been equipment
that can tune them in without interference. Adjacent
channels have been totally forbidden, theoretically
because of interference, since the early days
of TV. This is such a widespread idea and policy
that "adjacent channel" and "adjacent channel
interference" are not just good descriptions,
they are technical terms used in the discussion
of how to keep TV stations from interfering with
each other. Note that FM stations, which are
spaced 200 kHz apart, still are not placed adjacent
to one another, so they are at least 0.4 mHz apart
in any one market.

Yes, 4 and 5 exist in many markets, because they
are not, strictly speaking, adjacent. As pointed
out, there is more frequency separation between
them than the average two channels. 6 and 7 are
also not adjacent, but we don't have a 6 and 7
here in Los Angeles, and I wonder if they ever
existed anywhere in the old days when VHF was
king.

So, doing a rough count of channels and taking
adjacents into account, the old system might have
given a market 7 VHF and 25 UHF stations, while
the new system would give us 7 VHF and 38 UHF.