Apple is with a short A
Table is with a long A
Bubble is with a short U
BOttle is with a short O
Needle is with a long E
Middle is with short I
It is on how you pronounce the words. It is roughly like this
..........................................A.......E.......I........O.......U
Short Vowel Sounds:.....ah......eh.....ih......oh......uh
Long Vowel Sounds:.....ey........i.......ay.....ow.....yu
By just reading the letters that way and having now idea how it is properly pronounced, you will not know whether it is long or short. But we use symbols to indicate the correct pronunciation of the words