1998 P Wide AM Penny Why Is The Second 9 In Date Thinner?

It is caused by Machine Doubling.
You can see the step like appearance caused by the
bouncing of the die.
A common occurance.

MD will reduce the size of the device it is inflicting - on raised devices. With incuse devices, the MD will increase the size of it.

The reduction of the second '9' looks like die polishing that weakened those devices. (There was also Machine Doubling going on post strike as well) Note the other motto devices that were over polished and weaken them as well with the removal of the Ridge Ring that was removed from the die, in order to rescue the die to be used for many more coins struck. Looks like a Wide AM. (These are the easier years {1989 and 2000} to find these. 1999 being the harder year to find them)