For the relative angle of two lines not to change in interpolation, either the angle must stay the same, or the ratio of both line lengths must stay the same.
It is better known when curves produce a kink in interpolation, as shown in “Designing Multiple Master Typefaces” (Adobe 1995, 1997), but is also true for lines.
The problem is that not only the slant of the letters changes, but also the top bar of the T gets fatter.