SQL Server has more than one way to pull pages in from disk for your queries. SQL Server can do a physical read of an 8KB page, or an extent of 8 of those 8KB pages.
SQL Server can also use the “read-ahead” mechanism to pull even larger chunks of data in from disk when you have a query that wants to read a lot of data — because just plucking one 8KB page or even 64KB of pages into disk isn’t super fast when you need lotsa pages.
But these terms get a little confusing when you’re changing between different diagnostic tools in SQL Server, because some of these tools include read-ahead reads in physical reads, and some don’t!
There is some good information here, so read the whole thing.