When you amend a tax return, it may change the tax calculated based on what changes you made.
For example, if you increased your taxable income (e.g. you forgot to include interest from a bank account), that may increase the tax you owe, thus reducing or eliminating the refund from your original return.
Another example: If you overstated your itemized deductions and are amending the return to remove or reduce an itemized deduction, this may also increase your taxable income and thus reduce your refund.