This is also my experience dealing with unions. Management is legally required to make concessions during collective bargaining, so anything that can be considered an improvement to mandatory bargaining topics (pay, benefits, or working conditions) gets held back for the contract negotiation.
In one case, management wanted to purchase sit/stand workstations for employees, but HR pointed out that this was a change to working conditions and should be included in the collective bargaining negotiation. However management had already started taking measurements for the tables, so the union knew what management was intending. During the negotiations, the union decided to try and call HR's bluff and refused to ask for the workstations as a concession. So in the end, the whole plan was scrapped so that HR would have concessions available for future negotiations.
In one case, management wanted to purchase sit/stand workstations for employees, but HR pointed out that this was a change to working conditions and should be included in the collective bargaining negotiation. However management had already started taking measurements for the tables, so the union knew what management was intending. During the negotiations, the union decided to try and call HR's bluff and refused to ask for the workstations as a concession. So in the end, the whole plan was scrapped so that HR would have concessions available for future negotiations.