But perhaps the bigger surprise was that it took so long for a senior cabinet member to realise how deeply unfair the Government’s cuts have been for the most vulnerable.
The cuts to the Personal Independence Payment, the ex-Minister argued, should not have been “placed within a Budget that benefits higher earning taxpayers”. In interviews since his resignation, Mr Duncan Smith argued that the Conservative Party risk “dividing” society and targeting the worst of the cuts at people who "don't vote for us".
The way Mr Duncan Smith presented this observation suggested that this was something new, something that only his shrewd political mind could have noticed.
But for six years, first in coalition with the Liberal Democrats and now as a majority Government, this tactic has been at the heart of the Conservative’s economic strategy.
Far be it from me to question the authenticity of Mr Duncan Smith’s conversion, but if it’s taken him this long to realise what George Osborne has been up to, he must not have been paying attention.
Since 2010, there have been countless examples of the comfortably well off getting a helping hand while the heaviest burden is left to those least equipped to carry it.
We see this at a regional level - as councils and police forces in the Conservative shires are given a funding boost while towns like Rochdale are stretched to breaking point.
We see it in business - independent shops on our high street are burdened with exorbitant business rates while multinational corporations get tax cuts.
We see it at a personal level - with the well off allowed to keep tax relief on pensions while hard working people in Rochdale are handed the bedroom tax and reductions in tax credits.
Iain Duncan Smith acted as head cheerleader during the Chancellor’s Summer Budget speech in July last year. When even he thinks things have gone too far, it is clear the Government needs a serious rethink.