As part of research into logistics, academics at the University of Derby asked more than 70 furniture firms how long it would take to deliver a suite, and every one gave the 42-day period.
Transportation, waiting for the right colour and a need to follow what competitors do were all reasons for the stock industry-wide answer, said Stuart Berry, a senior lecturer in mathematics.
"Furniture makers would each quote the same delivery time but this quote was made without taking any notice of workload," he said.
"They all said it would be ready in six weeks. It was a standard answer."
"No-one wants to promise delivery inside six weeks as they cannot deliver but to quote any longer would risk losing custom to a rival."
In the same study, about 100 glaziers, or window fitters, quoted an average of four weeks for an installation date.
Berry and fellow academic Val Lowndes, who carried out the research as part of a post-doctoral study into logistics, believe the findings can be explained by a branch of maths called "fuzzy logic".
The core concepts of fuzzy logic date back to the work of Polish mathematician Jan Lukasiewicz in the 1920s.
It enables computerised devices to reason more like humans and can help explain why such trends in industry exist.