You committed the budget in March. You find out in September.
You commit open-to-buy in March against a spreadsheet and a feeling. Your supplier's forecast is a sales target wearing a forecast's clothes. You negotiate hard on margin in January and hand it all back in markdown in September, and everybody calls that the cost of doing business. Your supplier brings an analyst to that meeting. What do you bring?
What changes on Tuesday.
A forecast that thinks: seasonality, trend, paydays, promotions, and actual lead times rather than promised ones. Cannibalisation modelled before you commit, not discovered in the post-mortem. A lifecycle profile, so you know the shape of the curve before you buy the top of it.
The proof: Promised lead time versus actual, decomposed by stage. You will find out which supplier has been lying to you politely.