Incorrect.
The estimates are actually based on pretty hard data, ranging from electronics to visual confirmation.
On some days we may catch 70%, while on others we may only get 10% or less.
The Agents on the ground know how many get through. We used to keep track, but management and Washington saw the astronomical numbers and nixed that record keeping.
None of the estimates I've seen do that. It's always the same story: we caught X and since we only catch 10% of them that means 9-times-X got through. There's that built in assumption every time, and I've never seen it backed up by anything to prove they're still only catching 10%. They might have done some studying 20 years ago but now it's taken as a give that 90% get through.