Pedro Martínez: I don't know how much you're familiar with technical phonology terms, but /t/ is a voiceless alveolar plosive while /d/ is a voiced alveolar plosive. The place of articulation in both of them is the alveolar ridge and therefore, a switch from one to the other is quite simple. The only difference is in one of them being voiced, and the other a voiceless consonant.

So I think there must be a rule. Why don't they say the /t/ sound in 'better' in a way close to /z/ for example?