My understand was that it isn't actually legal for an acquirer...

My understand was that it isn't actually legal for an acquirer to be buying shares on market, but I may be wrong.

However, where the acquirer is paying a broker handling fee on acceptances, as they often do, it then becomes worthwhile for brokers to buy on-market at the offer price and then deliver them into the bid - the broker gets to keep the handling fee paid by the acquirer as their incentive.