Why are voiced and unvoiced consonants both acceptable for the same word?


It's common enough to see two possible pronunciations of a word in Japanese.  
For example, take the word **容態** (meaning 'condition' or 'state').

According to 三省堂 スーパー大辞林, both `ようたい` and `ようだい` are acceptable readings of
the word. I notice that this happens a lot with names too - for example 中田
being read as either `なかた` or `なかだ`.

What is the technical reason that voiced and unvoiced variants are both
acceptable?

