It's called lack of liquidity... and it's not due to a broker "out to steal your money", but rather a lack of trader participation in the market that causes the bis/ask to widen.
I guess you answered your own question…but today’s move of the GBPUSD versus EURUSD negates your “risk on/off” hypothesis as evident by the falling EURGBP (i.e., GBP is moving stronger than the EUR against the USD).
Best,
Dave
EURUSD x (1/GBPUSD) = EURGBP
So, if the EURGBP is climbing, this means that the strength of the EUR versus the USD is greater than the strength of the GBP versus the USD. Likewise, if the EURGBP is falling, the strength of the GBP versus USD is greater than the strength of the EUR versus USD...