Obviously the wall has to be designed for the load coming onto it. For a bearing, we can take an enhanced factor locally, the actual value of which depends on the position and orientation of the beam.
However, BS5628 also states that the wall should be checked for normal stresses 0.4 times the height of the wall down from the bearing. This means that the localised load can spread out somewhat, the extent of the spread depending on the position of the beam and the length of the panel (or the length between any openings, recesses etc that might interfere with the load spread). It's assumed that the load spreads at 45 deg.
So if the beam is positioned 2.5m above floor level, the stresses are checked directly under the beam (or padstone) and then again, 1m down, allowing the load to spread out, but not taking the enhanced local bearing factor into account.
The reason three courses of stronger blocks won't help in that respect is that three courses of blocks won't come down 1m to where the additional check is carried out.