Last week, Treasury was partisan, couldn’t understand Coalition policies and was no better than a mid-tier accounting firm.

Today the opposition is very happy with Treasury, but just has some “differences of opinion” with it on costings. Just some minor disagreements over some trivial issues between good friends, discussed in an atmosphere of respect.

And anyway, as Mr Abbott, the man who remains more likely to become prime minister shortly said this morning, economic competence isn’t about things like costings.

That line was too much even for my, I hope highly-developed, capacity to accept political hypocrisy. We all know Tony Abbott is an economic lightweight, but is there any need to confirm it with comments like that?

So for those who came in late, here’s the story so far: the Liberals thought it would be a great lark when they were in government to establish a process called the Charter of Budget Honesty that would trap oppositions into being forced to either be humiliated when Treasury found flaws in their pre-election costings, or dodging the process and looking like they had something to hide.

Like most parties when new in government, they didn’t think about the possibility that they might not be in government one day.

Come their turn, they opted to look like they had something to hide. Which, it turns out, they did.

And thus Peter Costello is presumably having an extended laugh today at the expense of Tony Abbott and Joe Hockey as their convoluted efforts to avoid the Charter trap have made things a whole lot worse in the eyes of the three voters that now matter, the rural independents.

For all the talk of a $7-11 billion black hole, it pays to keep a sense of perspective. Some of the “disagreements” are fair enough. The Coalition was not able, given its resources and lack of access to the Public Service, to accurately cost its expansion of education tax refund. And it was caught out by the structuring of tax payments between financial years between 2012-13 and 2013-14, which notionally made its PPL scheme more expensive. As I’ve said repeatedly, there are some things you just can’t know when in opposition.

That’s not to say that if Labor had made the same errors in opposition they wouldn’t have been pilloried by the Coalition and the right-wing media, but that’s just ordinary double standards. I can’t imagine the media letting Labor get away with saying 95% of its costings were right, which the Coalition used in its own defence last night. Hell, a 2.7% complaint rate for BER projects has been the basis for an extended campaign about the “incompetence” of Labor.

But some other “disagreements” are more serious. The $1 billion costing of savings to the Pharmaceutical Benefits Scheme — the best savings policy the Coalition came up with — was negated because the government had already factored in exactly the savings claimed by the Coalition. The costing of the Coalition’s employment policy actually broke Charter of Budget Honesty rules by assuming second round job creation effects.

That was worth nearly $1 billion over four years. And the Conservative Bias Allowance adjustment looks more than ever like a fiddle. Treasury and Finance’s comment “the Departments’ best estimate of a prudent Conservative Bias Allowance remains that published in the 2010 PEFO” suggests that it clearly isn’t regarded as prudent. Moreover, if that saving boosted the Coalition bottom line by $2.5 billion, it could do the same to Labor’s, taking Labor’s fiscal impact well ahead of the Coalition’s.

The Coalition insists it has worked hard on its savings and it will produce a better fiscal outcome than Labor. And there is some evidence to support that. But all that is obscured in the Coalition’s utter hypocrisy about the Charter of Budget Honesty and its fear of legitimate scrutiny. Politicians can’t just agree to disagree with Treasury on fundamental fiscal matters, not unless they want to to sit down with their staffers and write the Budget each year themselves.

Tony Windsor is right to be concerned about what he’s learnt from Treasury and Finance.