When I type this sentence into Word, the program dutifully underlines it in green and suggests: 'John parked the car.' That would be fine if John had parked the car, but what if I meant that the car was physically parked near John? My admittedly dated version of Microsoft Word (Word for Mac 2011) is programmed to recognize and correct passive voice, a no-no in most grammar circles. Les Perelman, a retired MIT professor and former associate dean of undergraduate education who ran the university's writing program, gave me this one: 'The car was parked by John.' That's why certain English sentences are such a pain in the neck for automated grammar checkers. English grammar, on the other hand, contains a near infinite number of possibilities, and whether something is grammatically correct or incorrect can largely depend on subtle clues like context and inference.
Spelling is a finite task with discrete right or wrong answers.