Resolved Printer inks.

I purchased recently compatible cartridges for the Epson printer.
I must confess buying compatible cartridges is my first, and therefore comments from other Posters in this field is most appreciated.

My question : In handling the cartridge or rather a gentle shake, I can hear a swish sound similar to a liquid in a container!!
I would have thought printer ink would be more dense or thicker than water.
Is cartridge ink denser in Manufacturers own cartridges than sold under the compatible banner?, reason for reduction in price of in some case 60%