I bought a laptop with 128GB but says 103GB available?

I recently bought a Lenovo laptop which said it has 128GB in the store. I haven't installed anything on it and it says 75GB left of 103GB. Can anyone tell me why this is?

The operating system will be using some of the space.

The remaining space is being used by Operating System

Because there's already stuff that has been pre-installed onto your computer. Every computer and smartphone is like that. Some of the programs that it comes with can be un-installed if you don't want/need them but, some of them might be necessary for the computer to function properly.

You also probably have a restore partition ( a part of the hard drive where a copy of the OS is kept in case you need to do a factory reset )…

Some memory is taken by default if you format your hard disk or flash drive into any format (NTFS, FAT32, EXT4 etc). Other than that, some companies keep some memory for recovery partition. So it may be the memory you lost.

Reset your windows or contact to the seller

The OS (Operating System) takes up some space, also manufacturers like to preinstall apps on the laptop, and that can take up some space, these things can't be uninstalled, and so limit the space on the Hard Drive/SSD

Because 1k=1024 bits, not 1000 bits.