There are various issues that enter into calculating the number of IOs. If the index covers
the query (i.e. all the columns that are required for the output are part of the index), then only the index page/row needs to be read.
If a clustered index is used, then with Sybase 11.9.2 and later if the table uses "all pages" locking then the leaf index page and the data page are one and the same, but if the table uses "data only" locking (commonly referred to as DOL) then the index leaf pages and the data pages are separate.
Which all means that estimating the number of IOs for a particular query can be a lot of fun, and is the reason why cost-based optimizers (such as the one Sybase uses) are pretty complicated beasts...