Here's why:
* The "ea" combination in "reading" makes a short "e" sound, like the "e" in "bed" or "pen."
* It doesn't follow the typical long vowel rule, which would make the "e" sound long if it were followed by a single consonant and then a silent "e" (like in "read").
So, "reading" has the short vowel sound, similar to the word "red."