Well, let's say each hair takes an average of 3-5 months to grow back, average being 4 months. Now, say the average person has 100,000 hairs on their head to begin. Let's say, in this theoretical test scenario, they start out with 100% anagen hairs, all are growing. Now, say they start shedding the higher end of the average # of hairs per day, 100. By 4 months (roughly 120 days), they will have lost 12,000 hairs before the first 100 hairs start to grow back. That means that at any given time, it can be assumed that this person has roughly 12,000 dormant hairs, which makes up 12% of their overall hair density. This is very close to the projected figure of 10% of all hair being dormant at once.
If you're losing 250 hairs a day on a regular basis, assuming they ALSO grow back after 4 months, you will be missing 30% of your overall hair density at one time, which is alot to be missing. This problem is compounded by male pattern baldness where the hair takes longer than 3-5 months to grow back, or may never grow back at all.
Edit: It's worth noting that in some cases, people may regularly lose 250 hairs a day. However, their hair is probably not dormant for quite as long as 3-5 months, maybe 2-3, and they probably started out with a hair density nearing 150,000 hairs, in which case 250 a day would not equal a net dormancy of 30%, but closer to 10%