Good day, colleagues!

I have one question about LOS 9g (Schweser): Construct a binomial tree to describe stock price movements:

“A binomial model can be applied to stock price movements. We just need to define the two possible outcomes and the probability that each outcome will occur. Consider a stock with current price S that will, over the next period, either increase in value by 1% or decrease in value by 1% (the only two possible outcomes). The probability of an up-move (the up transition probability, *u*) is *p* and the probability of a down-move (the down transition probability, *d*) is (1 - p). For our example, the up-move factor (*U*) is 1.01 and the **down-move factor ( D) is1 / 1.01**. So there is a probability

*p*that the stock price will move to S (1.01) over the next period and a probability (1 - p) that the stock price will move to S/1.01.”

I can`t understand, why the **down-move factor ( D) is 1 / 1.01,** not 1 - 0,01=0,99?