Here's why:
* Short vowel sounds are typically quick and sharp. They are often represented by the vowel letter alone (e.g., "a" in "cat", "i" in "pin", "o" in "hot").
* Long vowel sounds are typically longer and smoother, often marked with a silent "e" at the end of the word (e.g., "ate", "kite", "hope").
The "a" in "lady" makes a short vowel sound, similar to the "a" in "cat".