That’s because ALDI doesn’t cushion cost increases or sell loss leaders. If eggs shoot up in price 400% they immediately raise the price to match most grocery stores will try to eat at least some of that cost for some time hoping it will go down before they have to raid either further. That kind of pricing model means they need much larger margins on all their other products to afford that. Same way they sell milk and rotisserie chickens at a loss to get people in the store.
ALDI does not play those games and keeps their margins more consistent but their prices are more susceptible to spikes in costs.