It isn't that the Manchester sentence was a harsh one, but that the Heathrow one was on the low side.
In both cases the judge listened to the advocate's mitigation and made his decision. Plenty of people thought that the latest case would attract 12 months or even more.
What is clear is that both sentences were intended to be deterrent, and no pilot or engineer should be in any doubt about what will happen in any future cases. With the limit set as low as 20mg in blood, 12 hours' abstinence is no longer safe if you have drunk more than a small amount.