The percentage value on offers (xx.xx% above/below market) shows the difference away from the CoinmarketCap.com (CMC) rate.
If you set your offers using any other market (i.e. Binance), this figure will constantly change as all markets are independent and fluctuate in comparison to each.
It is important for all margins shown to other users to be in relation to a single rate so they can accurately compare different offers. Also, as some markets have consistently higher or lower prices than others, showing the margin in comparison to that specific market (vs CMC) would allow users to artificially deflate their margin percentage.