It really all depends upon who is talking, and what they are trying to convey.
Exactly: 1 megabyte is exactly 1,048,576 Bytes.
See, that is about a million.
Your original question "is 1350MB is that 1.3GB or not"
has the answer is "Yes" "Almost" and "Close enough"
Now to explain,
computers count in binary numbers (two values, a one and a zero), and it so happens that if you keep doubling 1 as you add more and more digit places(that's basically what happens when you count in binary numbers) you will find that with 10 digit places (i.e. 1111111111) that value is 1,023
(Another way to look at it would be if you assign the decimal "weight" to each of the successive binary digits, you keep adding to the left of the initial "1"
That value keeps on doubling. So the first 1, the one on the far right it worth 1 in our regular decimal system. The next (to the left) 1 is worth twice that or 2, the next 4, the next 8, etc.
So perhaps you can now see that a binary number, like 1111, is worth (8+4+2+1=) 15.
In detail, here are binary to decimal equivalents
You can keep on that same progression until you have twenty "1" characters and that value would be 1,048,575.
BUT ALL THAT IS TOO HARD, too convoluted, to try and keep straight in almost anybody's mind, and besides the exactness of it really is not that important, to most people anyway.
So, to keep things simple, most people speak in "almosts" or "abouts" as in
A Megabyte is about a million bytes.
A million bytes is almost a Megabyte.
And a billion bytes is almost a Gigabyte.
Where you will find the greatest disparity, though, is between the drive manufacturers and the operating system counting schemes.
Drive manufacturers like to use "almosts" because it is to their advantage to be deceptive, and use the approximations.
Computer counting systems though, have no trouble in being EXACT.
Drive manufacturers like to use the "almosts" to say that their 60,000,000,000 byte drive is (almost) 60 Gigabytes.
But since a Gigabyte is actually, exactly (1024 x 1024 x 1024=) 1,073,741,824 then a drive with 60 billion bytes is really just 55.879 Gigabytes (close enough, any of you geeks out there, who want to raise issue with that please say it in "Speakeasy" ).
So, how do you think the drive manufacturer like to refer to his drive? They will say it is (almost) 60 Gigabytes!
Of course, the computer you plug that same drive into, and the software that displays the real exact number of bytes is going to tell you 55 Gigabytes, and while the numbers are not the same, and it sounds like someone is lying (and they are, but it is supposed to be a "white lie").
So, a Kilobyte (1024) is almost 1,000 bytes,
or a 1,000 bytes is almost a Kilobyte.
A Megabyte (1,048,576) is almost 1,000,000 bytes,
or a 1,000,000 bytes, or a million bytes, is almost a Megabyte.
A Gigabyte (1,073,741,824) is almost 1,000,000,000 bytes,
or a 1,000,000,000 bytes, or a billion bytes, is almost a Gigabyte.
The next step ...? and this is coming in drives, eventually, is the Terabyte
or exactly 1,099,511,627,776, or almost a trillion bytes.
And repeating the original answer, "Yes" "Almost" and "Close enough"