But could that be due to just paying the lowest tiers more and highest tiers less?

Companies usually make salaries opaque, but say you have 9 software engineers. Your 3 underperformers are being paid $10 per year, your 3 average performers are being paid $15 per year, and your 3 top performers are being paid $20 per year. So average worker pay is $15 per year.

Now a union comes in and over the years the pay structure changes so that the most senior employees make the most, not the top performers. Now 4 youngest employees make $14 per year, the 4 middle employees make $16 and the most senior employee makes $18 per year. So average worker pay is now $15.3 per year, and you can now make the claim "workers in unions make more money (on average)", but I would argue this new structure is overall worse for the company since you are basically rewarding underperformers and punishing top performers in order to raise wages.