This has nothing to do with Parsec or Decimal. Two things are happening:
First, the type of the literal 3.14 is Fractional a => a. GHCi's extended default rules [1] pick Double as the default for Fractional. (This is a good choice.) Second, the closest number to 4.14 that is reresentable as a Double is 4.1400000000000001. This is an inherent limitation of the representation used. This effects every use of floating point numbers in every programming language, and every programmer should understand this. See [2] for more information.
For an exact result, you can provide a different type for your Fractional literal that is capable of representing the result exactly:
> succ 3.14 :: Rational
207 % 50
> There must be some reason why Float and Double implement the Enum class,
> but I can't understand in what sense (mathematically) are real numbers
enumerable
You're absolutely right. It is, at best, a kludge and has no mathematical justification.