There are hundreds of touch point sizes.
This isn't a problem if you build in a fluid, flexible manner. That is illustrated in this diagram. If however, you tend to build with very specific breakpoints and hard values for typography, sizing and spacing, you might find that even with the best intentions, you’re not providing the optimal user experience.
A viewport is the window size of the browser, not the screen size.
The main point we’re trying to get across is that you simply do not know how users are going to visit your website or web app. Instead of making design decisions on strict, limited breakpoints, keep in mind the sheer amount of fragmentation there is in viewports.