Calligraphic origin. The characters on the top line have a different construction than the characters on the bottom line. They have a different calligraphic origin. It doesn't matter if a typeface has serifs (like Times New Roman) or not (like Arial). It's about the original way they where constructed.
图中上下两行字母有着不同的结构，它们来源于不同的书法原型。不管这种字体是衬线字体（如Times New Roman）还是无衬线字体（如Arial），它们的架构都源于它们的书法原型。
The characters in the top line are constructed with a pointed pen (calligraphic tool). The contrast is caused by changing the pressure on the pen, not because of the form of the pen. Bodoni is one example of this, but also sans serif faces like Helvetica have this origin. The thickest part will be (mostly) totally vertical. From this perspective there is no difference between Bodoni and Helvetica. They both have the same construction. Only the contrast varies.
The characters in the bottom line have a origin which is derived from the broad nip. The calligraphic pen itself has a thick and a thin part. The contrast in the type is made because of the form of the pen, not because of the pressure. You slant the pen with an angle of 30 degrees on the paper. In that way your thickest part of a character will not be on a vertical direction, but will be on an angle. Also the thinnest part will not be on the most horizontal parts. Typefaces like Garamond and Minion have this kind of construction. But also sans serif faces like Gill Sans have a construction which is originally derived from the broad nip.
下面一行字体来源于一种笔尖扁平的钢笔。这种书法钢笔正面宽而侧面窄。字体的笔画粗细是由于钢笔本身形状的差异造成的，而不是笔压。你书写的时候笔身是倾斜的，和纸张形成30度的倾角。因此笔画最粗的部分就不是垂直方向的，而是有一定角度的倾斜。同样的，笔画最细的部分也不是位于水平方向上。Garamond 和 Minion 字体就是这种结构。但象Gill Sans 这样的无衬线字体也同样是起源于这种笔尖扁平的钢笔。