Not having access to the chart, I can only presume they got lower minimums with
a stepdown fix. The LOC trapezoids are quite narrow in close, so such a low MDA
is possible at a place where it is flat, close in. The MDA in theory could be
as low as 250 feet if there were no obstacles higher than the TDZ elevation and
the addition of the basic 230 feet of required obstacle clearance to the MSL
value of the TDZ results in a number ending in 20-foot increments so it doesn't
need to be rounded up.

David Megginson wrote:

I've just noticed in the latest CAP (Canadian approach plates) that the MDA
for the LOC(BC) 25 approach at Ottawa/CYOW has been lowered from 800 ft (427
AGL) to 660 ft (287 AGL). I can see how it makes operational sense -- it is
the approach runway usually named in the ATIS (despite the fact that two
others have ILS's) -- but an MDA of 287 ft AGL seems low for a non-precision
approach. Are there many similar examples out there?

All the best,

David