KIT day pay, how can this be correct?

[kitpaydrama]

I work for a local authority and earn 32k pa. Last month I did my first KIT day at 7.5 hours. I was paid £64. I queried this with HR who told me that policy states they deduct your SMP from your KIT day pay. Had a Google and it seems they can indeed do this. Then I queried why my day rate was £89 when I get over £16 an hour. They told me that your day rate is based on calendar days, so your yearly salary spread over 365 days. I don't understand how they can do this, obviously when I'm not on maternity leave I am not paid £11 an hour so why does this suddenly change?
Any advice very welcome. Feeling very angry as I've worked here a long time and put in a lot of unpaid hours over the years plus answering work questions on my own time on maternity leave because no one seems to have been able to competently pick up my work! Suffice to say that will be stopping now.

I work for the NHS. I worked KIT days during my first mat leave and they deducted a whole week of SMP off my wages. I was lucky that my salary for one day was more than my SMP or I would have earned nothing. I took out a grievance on the basis that I was not informed properly and I won on the basis that the average person would not be aware that SMP can only be paid in weekly amounts. My health board mat leave policy at the time referred to a KIT policy which didn’t exist so all that my manager could rely on was the national agreement which isn’t very clear.
However, they used a normal day salary to work it out I.e. 7.5 x my hourly rate.
There is now a policy in place and the general advice is not to work KIT days unless you are in the unpaid portion of mat leave.
It should be noted that employers do not have to deduct SMP but NHS salaries are legally determined so they have to do so. Your LA may be the same. (SIL got fully paid for KIT days, even when she only popped in for an ho or so to check emails she got a full day of money on top of SMP. I was/ am v jealous!)
Ask to see the SMP policy. If you were (like me) poorly advised by line manager what you would get paid then you can take it further. I couldn’t get extra money but they have me annual leave.
The 365 day thing sounds bonkers.

The 365 day thing is correct and relevant for all salaried members of staff in most organisations. Also kit days can be decided from SMP so sounds completely correct.

But why over 365 days? I am not contracted to work 365 days, I'm employed to work 261 of which 30 are paid holiday.
Surely this should be the denominator?
(Not the OP, but just asking)

I had no idea about KIT day pay, just assumed it would be at my usual rate of pay. Just spoke to ACAS who said they can pay what they like for a KIT day as long as its over NMW and stated in their policy. So looks like it is all legit
Thanks for your responses

@LostInTime yes I agree, I'm not sure why suddenly my day rate is calculated differently purely because I'm on maternity leave.

@gingerbiscuit19 what if you are not salaried? What other calculations is 365 days used for?