Most of what we refer to as "reciprocal" changes are not reciprocal. Strictly speaking a reciprocal change is the mathematically opposite patern in a lead oriented 180 opposite the lead in question (just the same as if you switch the left arm and right arm leads, lead 1 becomes the mathematical opposite of what its supposed to be). When we talk about tall R wave and st-depression in v1 with a posterior MI - this would be a true reciprocal change in V1, relative to posterior leads. Most of the other stuff that's commonly referred to as reciprocal, such as inferior ST depression / twi in an anterior STEMI, probably represents independent ischemic changes in these leads rather than mathematical reciprocity.
