Without any further info it is likely related to the battery capacity. The 25++ is probably the standard battery that comes with the laptop. The 55++ is an optional upgrade for a larger battery with longer battery life which means the laptop will be a little more bulkier and heavier.
Quite an interesting question - I had to find out, it is a measurement I had never seen in batteries and I use a lot of exotic batteries in my other hobbies.
The number system 25+, 25++, 27++, 55+, & 55++ used by Lenovo have to do with Watt/Hours for the respective battery with higher number having more watt/hours and therefore more life under a given load.
Very different if you ask me.