Selling before 5 years-SIP related Question

Hello everyone,,
Workplace SIP and selling before 3 years / 5 years
Hi wondering if anyone can help and has experience with share incentive plans and selling before 5 years.

I’m early in my career with the company and have been doing share save and SIP since late 2017. I don’t have any intention to sell the shares anytime soon but just if I do need the money - or feel I need to leave the company I’d like to know my options.

I just cannot seem to understand what happens when I sell before 5 years.

So an example:

So was looking at my wage from September 2017 - did full £600 - but only paid £415 as that was the difference between the September wage and August wage.
That gave me 312 partnership shares and in October 2020 I get another 312 shares for free (matched shares)

So when I try to sell those 312 shares now - it tells me 312 matched shares will be forfeited. Then I sell the 312 - it’s £517 (share price has gone down a bit since then) but I pay £15 commission then £10 NI and £224 income tax so I only get back £268

I know the share price has gone down and there is the commission fee but can’t understand why I get back a lot less than I initially paid.

So initially I saved myself approximately 30 % and got shares worth £600. However I now try to sell those same shares and I could half the value back after tax, ni and fees. Obviously the answer is to just don’t sell them whatever I do until October 2022 but sadly can’t guarantee I’ll still be working there by then.

Anyone able to explain the difference from when I bought the shares to selling before the 5 years?