I was having the same issue and this is what my research revealed when you are sitting you are compressing your lower abdominal muscles that comprise your pelvic floor. Consequently when you do a Kegel exercise the compression and tensing of your muscle group holds the urine in the bladder. However, when you stand the body returns the muscles and abdominal organs to their natural relaxed position. Now when you perform a Kegel exercise the same muscle groups are working but you are not deriving the benefit of upper pressure from the organs and pelvic floor muscles.
Gravity is a factor but the real issue is the difference in position of the muscle groups and internal organs between sitting and standing.
This seemed like a logical explanation and as time as progressed the leaking while standing has diminished.