This is a bug
which has been around in Windows
at least since the days of Windows 3.1
and is still there
It can show up in any application using the symbol font, but is most common in Word's Equation Editor.
Windows comes with two different versions of the Symbol font, a bitmap version and a TrueType version. The bitmap version is coded so that Windows believes it is the non-italic version of the font, but the lower case characters in this font are actually italic. This is probably a case of Windows trying to be too smart for its own good, as it is typographical convention that in mathematical text, especially for equations standing alone, letters representing variables are printed in italics, except for capital Greek letters which are probably made different because so many of them are indistinguishable from our normal letters, and such variables are of course one of the biggest uses of the Symbol font.
Windows mixes these two fonts together in the way it presents them to applications. When an application wants a particular font for display on screen, Windows gives it a bitmap representation that is taken directly from a bitmap font if a bitmap is available in that size, or otherwise supplies one rendered from a TrueType font. Here's where the bug comes in, because Windows gives the italic lower case Greek letters to applications as the non-italic font, and they use it that way. You can see this yourself by double-clicking on the Symbol font (either one of them) in the Fonts Control Panel. The font viewer is just another application, and it suffers from the same bug. Notice how some of the lines are italic, and some (particularly the larger ones) are not? In other font displays, none of the lines are italic.
Some applications will make an italic font from this bitmap by simply staggering the rows of the bitmap. This produces the strange over-slanted effect. The Equation Editor does this, and it results in these hyper-italic letters in the default configuration of the Equation Editor.
The only solution I know of is removing the defective bitmap Symbol font from your system and having Windows use the TrueType font for everything.