Whatsapp - why are some messages blueticked at the exact same time?

Hi all

Would appreciate any insight into the below.

I was scrolling through an old chat with a person who has blue ticks switched off. I see that now the very earliest messages in the chat are double blue ticked, with the latest ones remaining double gray ticked as usual.

Clicking on the info tabs for these messages it says they were all read at the exact same time month say 13/12/21 at 14:58 pm (even though the messages are 1-2 years old and part of a conversation we were both responding to so I know they were read much earlier even though there used to be only two gray ticks).

I also noticed that even my voice-notes that used to have two gray ticks to show they were delivered but not whether they had been listened to, now have two blue ticks. I know they were being delivered and heard as I used to get responses specific to the content of the voice note. When I look at the info of those messages I see that all the voicenotes were heard at the same same date and time as above.

What's odd is that after the first 6 months or so of messages, the written messages go back to being gray ticks, but all the voicenotes now blue ticked whether older or recent.

What causes this? To cause the first say 6-8 months of old chat messages to suddenly become blue ticked and say they were read last month and the rest stay gray ticked? And more confusingly, what causes every voice-note from past to relatively recent to become blue ticked and say they were all heard simultaneously on 13/12/21 at 14:58 ?? I am really puzzled and would appreciate it if anyone has a clue!