Style-Device width details
Having initially developed the mobile level screen layout using the Samsung S4 phone resulted in a nice layout, but then when viewed on an iPhone everything was too wide, so another 20% had to be trimmed off the designs. This results in some variation on style between phones, but is a reasonable compromise. The tablet level also has similar variations between devices, and layouts vary widely when a device is rotated between vertical and horizontal. Traditional fixed width layouts do not work well on mobile devices, but the current framework provides a workable format today.
If at some point a better mechanism to identify the target devices resolution and layout and then style appropriately, then the theme system can be extended to support that without having to change any content.
Device resolutions are always changing, and the trend is to higher resolution devices which Graphics Display Resolution on Wikipedia documents nicely. What does muddy the water a little is that many small mobile devices automatically scale fonts and images by a factor to maintain readability on the small screen so although the screen may be 900 pixels wide, it looks like 300 pixels to the software, hence the selection of that size for the small screen devices. A pixel is not a pixel is not a pixel is a little out of data but provides a nice explination of the process.