Bid-ask spreads for UCITS ETFs holding US equities

Conventional wisdom dictates that the bid-ask spread for a UCITS ETF traded on the LSE, that holds primarily US equities, should be lower when both the LSE and NYSE are open, and higher when only the LSE is open. I, however, found this was not the case for the AVGS UCITS ETF, around 70% of whose holdings are US equities.

I recently switched from holding AVUV and AVDV in the NYSE, to AVGS in the LSE, to avoid estate tax implications in the US, and to take advantage of the accumulating nature of AVGS to minimise tax obligations at home, in New Zealand.

This was the first time I traded on a non-US stock exchange, so I actually woke up at 01:30 in the night in NZ (the start of the period when both the LSE and NYSE are open) to purchase AVGS, but was disappointed to find the bid-ask spread, as quoted by IBKR, was actually larger then. It was around 0.08% when only the LSE was open, but around 0.15% when both the LSE and NYSE were open.

So I ended up making subsequent trades at more humane hours in NZ when just the LSE was open. Did anyone else have any similar experiences, and have any explanations for this?