R12.1.1 Invoice validation cause different base_amount for each item line and tax line

1.Before invoice validation, create invoice with following amount:

Line    Amount        Base_Amount
1item     90.91             1970292
2item     90.91             1970292

2. Click calculate tax button:

Line    Amount        Base_Amount

1item    90.91             1970292

2item    90.91             1970292

3tax line 19.09          197008
4tax line 9.09            197008

Till here all data was correct.

3. After validate invoice by click actions>Validate, the base amount was changed as below:

Line    Amount        Base_Amount

1item    90.91             1970293

2item    90.91             1970292

3tax line 19.09          197008

4tax line 9.09            197007