The first part (five steps) is based on the fact that HM doesn’t report MAP numbers until the start of the sixth step. I admit that I am making the assumption based on this that it needs at least five minutes of ramp data to calculate a MAP.
The second part (fail the ramp) is based on the fact that the test expects you to fail. If you don’t, then you won’t have reached your maximum power levels, so the calculated MAP will be too low.
I wasn’t able to quickly find any official documentation about either of these points. So everyone should take what I’ve said with a suitably sized truckload of salt. 