The Bollinger channel boundaries are not fixed and depend on the volatility of a financial instrument.

If there is no clear-cut trend, the price deviates slightly from the midline. As soon as the trend gains strength, the deviations become larger, and the channel's boundaries diverge.

Bollinger Bands have a useful feature: the price stays about 90-95% of the time within the channel.

Buying in an uptrend, if the price has tested the middle line from above and is heading towards a trend;

Selling in a downtrend, if the price has tested the middle line from below and is heading towards a trend.