Calling a portable computer anything but a portable computer is simply marketing. Why is one called a laptop computer when using it on one's lap for any length of time is likely to cause burn marks on the legs and other vital organs?
The computer industry has some vague sizing notion about naming portable computers, so you have ultra-mini, netbook, laptop, notebook from smaller-to-larger, but there is no definitive point at which one becomes another. Is a Dell 11" portable a "netbook" or a "laptop?" Or a notebook? Again, it's really a matter of marketing. And a not-very-accurate way of segmenting the market.
Incidentally, did you know that early PCs were called "microcomputers?" That's because when they came on the scene there were mainframes and "mini" computers (that took up less than a roomful of space). Out of this evolved a company that created software for this new market segment: Micro Soft (its original name before they combined the two words and grew it into the world's largest software company).