Here’s the logic…
If students were to spend 25 minutes a day reading at a rate of 200 words per minute for 200 days out of the year, they would encounter a million words of text annually. According to our estimates, with this amount of reading, children will encounter between 15,000 and 30,000 unfamiliar words. If 1 in 20 of these words is learned, the yearly gain in vocabulary will be between 750 and 1,500 words. Nagy and Herman (1987 p.26)
Researchers estimate that of every 100 unknown words we encounter during reading, approximately 5 will be learned for their meaning (Nagy et al., 1987)
Although this appears to be a small proportion, students who encounter 100 new words each day and red every day of the week could learn 35 words each week and almost 2,000 words each year.