Disparity between joint and sole assessments

We are jointly assessed for tax. I was a PAYE worker. My wife is self employed

I was a PAYE worker up to 31/12/2014. My P60 shows final income of 76,979. Total pay for USC purposes is 78,596.
Tax paid 17,833. PRSI paid is 2,736. USC paid is 4820.

My wife is self employed. Her nett profits after EXPENSES is 10,474 for 2014. Her capital allowances are 1030 (all for 2014). Our health expenses tax credit for the year came to 120.

Our accountant worked out that our extra combined tax bill for 2014 was 2264 (including my wife's minimum self employed PRSI of 500)

However, I worked out myself what my wife's tax bill would be as on her own, and it works out at €232 tax and PRSI of €500. Why is there such a disparity between the joint amount we owed, and what she owes for her self employed income ?

Is the difference attributable to my tax situation ? As PAYE, I would have assumed that my tax was taken correctly at source

I can't figure it out. Any guidance would be much appreciated