daydreamer65’s answers should mostly suffice. I would say only correct one thing. There is no “on-exchange” spot forex trading. The only forex trading which can be done through an exchange (at this point anyway) is via the futures and options market.
What is the cause of this difference? I took both samples at the same time.
Ans 1 Different brokers have different bid/ask spreads.
Why? (30 Pips is a pretty big difference).
Market makers set their own prices, that is the nature
of the beast.
Second question,
Why has the spread on Oanda’s USD/JPY expanded to 10 pips over the weekend (from the usual 2 pips)? Is this standard practice?
Ans 2 Yep.
Why?
The spread of any currency pair is based on the liquidity available on the open market. When the markets close in the evening and on the weekend OANDA still allows you to trade, but must increase the spread to accommodate the risks associated with high volatility and low liquidity.
Spreads are generally tightest during the overlap of trading hours among the major FX market centres: London, New York and Tokyo. For non-major currencies the market hours of that nation impact the spread as well.
Taken from the Oanda site.