God knows how the OBR does their sums, but by my research the public sector in the UK employs about 6,000,000 people, the annual staff turnover is 12.6%. So every year 756,000 people leave the public sector. Just by not replacing folk you're going to lose around 3,000,000 folk over six years, without mass redundancies.
Just by not replacing people you could account for the OBR figure in less than a year.
Sent using BlackBerry® from Orange