The font's creators wanted historical accuracy over what looks right to us now.
The model for the Garamond installed with Office, Monotype Garamond, is actually the work of Jean Jannon (misattributed to Garamond - long story). You can see a 1928 print of his original in this pdf on page 36, wonky italic capitals and all. nihilistenhymne's answer is not quite right - italic capitals had been introduced long before he started work. Why are his so wonky? I don't know, but my guess is that the idea was that the angles would help letters fit in with lower-case text, and the italic capitals weren't intended to be used in all-caps settings all in one go where the unevenness is obvious. Some italics from the time had less wonky characters - this one from 1571 is a bit more even.
It's not a mistake. Monotype's designers could have cleaned up the design if they'd wanted to - this was the same design team that did Times New Roman, which has very even capitals with constant slant angle in italic - they just wanted to accurately replicate the original.
There can never be one true "Garamond" digital font - it's not 100% clear what fonts Garamond did and didn't cut, those he did cut don't all look the same, and I believe that it's not completely certain that he ever did an italic at all (although he did print with one, so clearly he wasn't averse to them - see Vervliet's books on the topic if you're near a good university library). Basically, "Garamond" is really code for "it's based on French printing between 1530 and about 1620." So any modern "Garamond" font is an interpretation - what are you basing it on, and how much wonkiness are you going to keep? Some versions (Adobe's, Stempel's) go for an italic with regularised capitals, others don't.