Flight scheduling system test case: flight number re-use

Scenario:

• Mom books a flight from airline X, gets assigned flight number 123, arriving at airport YYZ @ 5:45am
• Mom tells son: “my flight #123 arrives tomorrow @ 5:45am”
• “Tomorrow” comes. Son goes online and checks flight info, which states that flight 123 is on-time.
• 6:15am: son arrives at YYZ to pick up mom.
• Son checks the Arrivals Fight Info board, which says flight 123 has landed. Son waits at the Arrivals Area.
• Three hours later. Mom was “no show”. Son calls mom, who says “oh, did I say ‘today’? I meant ‘tomorrow’. Tomorrow @ 5:45am.”

Why someone would design a flight number assignment algorithm such that any X flight departing XXX for YYZ at time hh:mm, on any day of the week, would get assigned the exact same flight number, is beyond me. Are flight numbers so scarce that they have to reuse the same numbers every day?