Unrepeatable reads: Assume T7 reads X twice during its implementation. If it did not update X itself it could be very disturbing to see a dissimilar value of X in its next read. But this could occur if, among the two read operations, another transaction modifies X.
T7
T8
Assumed value of
X=2000
READ X
2000
UPDATE X
3000
Therefore, the inconsistent values are read and results of the transaction may be in fault.