I found this:

The following equations have been developed by Ritchie (2008) for determining chlorophyll (Chl) a, b, c, d, and total Chl for 90 % acetone, 100 % methanol and ethanol solvents suitable for estimating Chl in extracts, using a spectrophotometer:

Chl a = – 0.3319 A630 – 1.7485 A647 + 11.9442 A664 – 1.4306 A691

Chl b = –1.2825 A630 – 19.8839 A647 – 4.8860 A664 – 2.3416 A691

Chl c = 23.5902 A630 – 7.8516 A647 – 1.5214 A664 – 1.7443 A691

Chl d = – 0.5881 A630 + 0.0902 A647 – 0.1564 A664 – 11.0473 A691

Total Chl = 21.3877 A630 + 10.3739 A647 + 5.3805 A664 + 5.5309 A691

You can also use a portable chlorophyll meter (e.g. CCM200) to estimate chlorophyll content index (CCI) without any plant destruction.

So try recalculating and hopefully it works, since total should always be the same or greater than a + b :)

Yes you can go through the attached article