What happened with Birmingham Pub Bombings?

Birmingham, England, November 21, 1974. 21 people are killed in a bombing attack by the IRA (Irish Republic Army). But some things don’t add up- at 20:11 warnings were given by IRA representatives. At 20:17 the first bomb explodes. At 20:27 IRA representatives warn authorities of a second bomb just as it explodes in a pub not far from the first bomb. A third bomb fails to detonate.
The IRA have been known to give warnings an hour in advance for civilians to evacuate the building (Manchester bombing, Victoria Station bombing, Bloody Friday.)
So do ya see the mystery? 6 minutes difference between the first warning and first boom. Obviously the place won’t be evacuated in 6 minutes. So why the phone call?