The RSI indicator uses data gathered from previous trading sessions to reach a reasonably accurate conclusion if a coin is being overbought or oversold. When you decrease the timeframe, the index has fewer data to draw its conclusions from. As such, the chart becomes more sensitive as the period decreases. In the same way, increasing the timeframe for RSI increases the amount of data that is factored into the calculation of the index. Thus, data becomes less sensitive as the period is elongated.