You said it yourself. UTC is a non-uniform time scale. "Actual Time" (as in UT1, for example) is, by contrast, a uniform time scale. SQLite3 (and most things) use a uniform time scale. However, there is a non-uniform scale called UTC which is used to approximate reality. Since UTC only approximates reality, it must be "corrected by artifice" periodically so that the imaginary aligns with the reality, and these "artifices of correction" are called leap seconds. Leap seconds only exist to make the imaginary consistent with the reality.