The data attribute is the RAW value stored by the disk, which has not been interpreted in any way. they reflect binary data, that may mean different things on different drives.
SMART analysis tools make assumptions about the data value when the convert the raw value into the Current and Worst values for analysis and display. Sometimes the data value 0x00 indicates a current value of 100, and worst of 100, which indicates that of the 100 instances before failure, you have used none of them. In the case of Reallocated Sector Count, the data counts down from the manufacturers determined threshold, but the current and Worst values count up, not down, until failure. It wouldnt make sense for you to see a value of 100 when you actually have 0 reallocated sectors, but see 0 when you have 100 of them.
Additionally, Since a SMART analysis tool like SpeedFan or SmartMon or palimpsest must understand the workings of many differant kinds of disks, their interpretation of the raw data may at times be inaccurate. The data field is displayed so that the user can take matters into their own hands in the case that the current metric is suspect.