You are correct, but the STFTs are an instant reaction to the voltages of the O2 sensor feedback. The LTFT are the averages of the STFTs.

Think of it this way, your car is tuned and happily driving at 14.7 when all of a sudden you suck a small baby into your intake. The ECU sees the O2 sensor voltage go way high and starts pulling fuel to keep 14.7 AFR. Not much air getting past the baby so your STFTs might be -15 or so. Now being that the STFTs can't adjust fast enough being choked down to -15 it applies a negative OFFSET (LTFT) and starts back at 0.

Ok, I'm tired and being silly, good night.