This was because the series 1 and most series 2 RPC 600 motherboards had RAM sockets which had a rather high back. Subsequently most PCs had lower sockets which enabled the chips on the back of a SIMM to be much closer to the edge of the board so the whole assembly could be much lower profile. However, this meant that SIMMs with chips on the back as well as the front and where the chips were very close to the edge wouldn't fit in the early RPC socket.
The vast majority of these were 32Mb, which are almost always double sided. Hence when 32Mb was the most common RAM upgrade for RPCs there was a problem in this area. Almost all 64Mb SIMMs have chips only on the front so the problem doesn't arise.