Two things you have to remember. One, the difference between the small b and the upper case B. Small b is bits, upper case B is Bytes, where there are 8 bits.
Two, ISP's offerings are always "UP TO . . ." X number of bits per second (notice I said bits, not Bytes). Downloading anything from any site is totally a crap shoot on speed. Some sites are fast, some are slow. ATM timing, backbone (fiber) sizes are different, and time of day will affect speed. Connect speed is measured in bits per second, download is measured in Bytes per second.
And, different PCs will show different speeds.
BTW, if you look at your first sentences, it's pretty close to correct. ISP offers up to 12 Mb/s (small b, bits), and you're getting 1 MB/s (Upper case B, Bytes). Eight bits in a Byte. Do the math.
And, in closing, you'll never get advertised speed. Period.
40 years teaching Telecom, analog and digital data, and transport.