I feel this is aajis planning. Before police takes virat, ammu says he is bekasoor , then look at ajjis expression. And then ajji reaches ps before amu and she suddenly gives this statement.
Thats one angle (and my initial thought too).
But can also be that Ammu does that cause she wasn't able to convince Aaji... and figured she cant convince others if that's the case.
Aaji reached before cause Ammu had to plan what to do? (btw why was Aaji even there? He is arrested, job is done... or to make sure he doesn't get bail?)

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Jail 