The standard @ symbol has four horizontal lines and worse the right side of the "a" is a vertical line contained inside a circle without touching it.

In a variable width font it's often fully twice as wide as a regular "a" character. The variable width font lemmy uses for example, at least as rendered by my computer has six pixels for a lowercase "a" and also six pixels for the small one contained inside the "@" symbol, then another six pixels of width for the circle around it.

That's an impossible task in a fixed width code font where users typically choose a size so small that the regular "a" can't be reduced any further while still being readable.

Which is why basically all code fonts (including Source Code Pro) cheat and modify the symbol so the inner circle overlaps the outer one on the right edge. Some of them do that better than others at inventing their own variant of @.