Why would a girl be lowering her voice when they see you?

[Greateye]

Is this a way to seduce you or appear more attractive?

Why would girls do that (instead of increasing their pitch)?

Context:

There's been many times when I started to talk with many girls. At the beginning of most conversations they start with a high voice tone (high pitch tone). After 3 or 4 conversations, on the next time we've encounter, their voice gets lowered (pitch tone gets lowered).

The difference is immediately noticeable and sometimes they show some signs of attraction.

[FilmGuy93]

It's just a way to be demure, gentle, and unassuming around a man they respect.

[aaron210419]

It could be her way of putting up a kind of barrier. In general, women tend to speak in a higher tone around guys she is interested in.

[Akam93]

My voice is always the same unless I'm being flirty.