There are two cases when this happens.
1. The two users might have logged into the app at different times. Since we provide a minute by minute update of the prices, they both might see different prices as their respective apps refreshed the prices at different times.
For example if user A logged into the app at 1:42:03 PM, they would see the latest price at that time. If user B logged in at 1:42:40 PM, they would see the same price as User A. But at 1:43:03 the price would refresh for user A, but User B would still see the old price since the price refresh for user B would happen at 1:43:40 PM. Hence at a particular time, both users can see different prices.
2. If both users own different quantities of the particular coin. For example: if User A has x amount of BTC and user B has y amount of BTC, the sell price shown to them is based on the quantity they own. Hence in this case as well, the sell price shown to each user might be different.