Scott,
Coincidence? Or perhaps just sound JIT inventory management. 🙂
Do you also notice all the times when you order a spare and don't immediately need it for a job, which probably outnumber the times when you do?
Are the spares you tend to be short of generally the ones you use most often?
If you've been doing this for a while your subconscious may have stored away typical usage patterns for various spares, based on the jobs you've done over the past few years. So when you 'randomly' notice an empty widget box in your van it may be your subconscious prompting you: "November, right mate we're due a few Worcester 200 heat exchanger AR5 rings soon as the sudden cold tends to perish them."
As Sheffield says we have a tendency to overemphasise what strikes us as significant and discount the ordinary, especially in memory, i.e. we love to seek patterns or 'meaning' in mere coincidence.