As always, you need to be very careful to understand the base (reference) when discussing percentages.
The experimental probability of getting tails is 48%. The theoretical probability of getting tails is 50%. When you take the difference of these numbers, you can conclude ...
The experimental probability is 2% lower than the theoretical probability
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Often, when we talk about something being a percentage lower or higher than something else, we use the "something else" as the base. Here the theoretical probability is 0.50, so the experimental probability is
(0.48 -0.50)/0.50*100% = -4%
relative to the theoretical probability. That is, one could also say "the experimental probability is 4% lower than the theoretical probability of obtaining tails."
