It's a control built into the system to avoid fraud... there would be an expectation that people shouldn't need to revisit the same tax year more than a couple of times i.e. you sit down, figure out your entitlements and claim them in one go. You might miss something the first time around, like some of your medical receipts, and need to go again, but not more than a couple of times.
By any objective yardstick of risk, you'd have to consider people adjusting their figures several times over (particularly where each adjustment leads to a further repayment), poses a risk. Better in that case to save people from themselves, and check what they're doing before you give them any more refunds...
Otherwise the Indo and other right wing media would be having a field day with our "lazy Civil Servants" laissez faire attitude to administering the tax system...