Right. So, 45 minutes on hold and 30 minutes on the phone with the IRS has led to the following explanation: The IRS made an error when processing both my 2013 and 2014 tax returns (apparently the difference was so small in 2013 that they didn't bother to tell me...nice one...).
But also, in part, it's due to a decision I made in 2008 (since I believed I'd be living in the UK for the foreseeable future) to change my tax year from the US calendar year to match up with the UK tax year (6 April to 5 April).
So, the IRS processed my last two tax returns as if the information were nearly a year behind (e.g., for 2013 they were processing it as if I had included my income through 5 April 2013, even though I clearly wrote '5 April 2014' as the end date), and thus they decided that I should have used the standard deduction and exemption amount from the previous tax year. When she looked closely at the end date for my last tax return (5 April 2015 - which I managed to submit on 15 April because I was so amazingly organised this year! :-) ) she agreed that the standard deduction and exemption amounts looked correct for 2014. So I don't have to pay anything (at least not yet) until the two tax years in question have been fully reviewed in a month or so. At which point they'll hopefully realise their mistake.
![Grin ;D](https://www.talk.uk-yankee.com/Smileys/classic/grin.gif)
Not a fun phone call, but I'm happy to know that - assuming the powers that be agree with the agent I spoke to - it wasn't my ability to subtract that was incorrect!
![Smiley :)](https://www.talk.uk-yankee.com/Smileys/classic/smiley.gif)