Futures and spot transactions are similar to forward transactions, but they are not exactly the same. For example, rb 1 105 indicates that rebar will be delivered in May of 201/year; Rb 1 106 is the delivery of rebar in 201/month.
As for the price difference, it is the price expectation of futures speculators for future futures varieties. Any variety has off-season and peak season, and the price in different months will of course be different.