You get £1960 for the Basic element and £2010 for the lone parent element of Working Tax Credits, which totals £3970.
You are allowed a £6420 income disregard, so £6991×0.41(41%)= £2866.31 would be taken off of £13331 or £6080×0.41(41%)= £2492.80 would be taken off of £10,000 (because you would be treated as having £12500 income, not £10000, because of the £2500 disregard for a drop in earnings).
They always start the deductions in the following order:
WTC work elements entitlement at the taper rate of 41%
WTC child care element at the taper rate of 41%
CTC individual elements at the taper rate of 41%
CTC family element at the taper rate of 41%.
So your WTC elements total £3970 and this year, if they aren't using in-year calculations for you (which they won't be, unless you've asked for them), they will be using last year's income to calculate this year's TC. So £2866.31 is deducted from £3970, leaving £1103.69, or £84 every 4 weeks. I think they are probably giving the £78 and holding back a few £ to allow for changes, which they often do.