There is no good reason. There are times when you would and times when you would not.
199 over 198 estimate = 1 actual answer would be greater than 1 35 over 17 estimate = 1 actual answer would be greater than 1
If you were a car mechanic and your actual amount came in lower than the estimate, your customer would think he or she got a good deal.
An upper bound estimate is a estimate that is greater than the actual solution.
67.125
When you round both factors in a multiplication problem up, your estimate will be greater than the actual product.
over-estimate
over estimate
An actual measurement is going to be more accurate than an estimate.
67.125
When the actual answer is not a solution that can be applied in real life and a higher number is required. For example, consider the number of coaches required to transport 60 children when each coach can take 40. The actual answer is 1.5 but you cannot get 1.5 coaches so the real-world answer is 2.
The actual yield is less than the theoretical yield.
Your estimate will be greater than the exact number.