Hello. Thank you for your enquiry. Our compatible black cartridge holds 24ml of ink and each colour cartridge holds 15ml. Calculations for the twin pack with 2 free black inks are as follows: 4x 24 + 6x 15= 186ml.
As for the taster pack you refer to, it is important to note that it consists of a standard set of 4 genuine HP cartridges and a set of XL compatible inks. Standard genuine black cartridge holds 6ml of ink, colours - 3 ml each, so that's 15ml in total. Our compatible set - 24ml black, 15 ml each of the colours, that's 69ml in a set. 69 + 15= 84ml.