you give hourly slots every day, 365 days/year where you are going to be from 6am - ?6pm
at 6am you are usually snuggled up warm'n'cosy in bed, but if, as expected, you give your 6am location that day as at home, very possibly tester will turn up randomly at 0600 on some day ( anecdotally 0600 seems very popular time for tester to turn up )
if your doorbell isn't working on that day they turn up, they try again & wait something like 1/2 hour for response - door opening
if they don't get door opening by then, it's a "missed test"
they don't even like waiting 1/2 hour, because, 1/2 hour is more than enough delay for a doper to self-catheterise with "clean" urine from another source or carry out the famous hungarian discus throwers of '04 trick of concealing a "clean" urine bag in the butt with tubing strapped to nether regions & underside of penis & pee into pot hoping tester who has to observe the urination with shorts down doesn't see it ( a big dick may be beneficial in executing this trickeration )
athletes don't have much of a spontaneous life...