What is the difference between `IO` and `Data` in the Activity Monitor?
Apple provides an explanation on their Support site:  


  The Disk pane shows the amount of data that each process has read from
  your disk and written to your disk. It also shows "reads in" and
  "writes out" (IO), which is the number of times that your Mac accesses
  the disk to read and write data.



Disk I/O is the number of reads/writes that are done (how many)
Data is the aggregate amount of data transfered (how much)