Think you meant 7 and 10... not 4 and 10.
As far as the packets of solution - even though the big bottles are cheaper per unit of fluid, the packets really make calibration easy. There's just the right amount of fluid in the packet to be able to fully immerse and swish around the probe, right in the packet itself. No need to waste fluid filling up a container to get it at the right level. Also, since you're using the packet itself to calibrate in, you don't have to worry about cleanliness of whatever you pour the solution in to - there's no chance of cross contamination effecting the calibration solution.
Just my nickel's worth.