I wouldn't tend to attribute any improvement to cable losses. 6 ft of RG400U would produce about 0.5 dB of loss at these frequencies which is near to nothing.
Reading the
Wikipedia article about how monopole antennas work, I'd suspect the improvement observed comes from the cable adding additional RF grounding capacity to the antenna footpoint. That said, any better grounding of the footpoint (min. 1 sqft. of sheet metal) should improve the VSWR. This applies as long as the antenna is properly tuned to the TX frequency. Metal parts near to the antenna rod can heavily de-tune an antenna.
If grounding and wiring seems good, it will take an antenna analyzer to find the root cause. You might find help from a radio amateur near to you, easily to be identified by the huge antenna tower in his backyard.