The AIP (ENR 1-7-3 5.3.4) states that when a runway threshold is 7 feet or greater below the aerodrome reference point (ARP) then a threshold elevation is to be passed to aircraft landing QNH together with that pressure setting. Cant speak for the other airports but the figures at Edinburgh are ARP 135 feet, threshold 06 110 feet, threshold 24 100 feet roughly 1mb difference in pressure at the thresholds than at the midpoint (where the ARP is). Rather than pass this information to each individual aircraft during what is already a bit of a mouthful when clearing to intercept the localiser, descend on the glideslope a decision was taken to instead state the threshold elevation on the ATIS.
Hope this helps
Baggy