The Black Hole of Home Improvement Stores

Why is it that, no matter how carefully planned a trip to Lowe's or Home Depot is, or how sure you are you measured everything you could possibly need to measure (because you finally measured all the things that you realized last time you were at the home improvement store you had forgotten to measure), or how sure you are you've thought of every project you're working on, that it is impossible - impossible - to leave the home improvement store without a list of follow-up tasks and a return trip in your future?