Confused about this... Can someone clear this up for me..

My current internet plan is the Fiber Internet 300mbps plan, it costs $80 a month, $75 with discount. But I noticed there is a higher speed of 500mbps for $75, but $70 with a discount. Why is the higher speed $5 cheaper? If I switched now will they charge me now or until my next billing? I'm confused why it's cheaper and if there's a catch. All of these plans are purely internet, don't require phone service.