You got a wait day when you were sick
When you have a fixed hours contract and were sick, you can get a wait day. This means you won't get salary on the first day you were sick. Therefor you have less salary this month.
You worked during public holidays last month
If you are a skilled worked (more than one year of full time experience in your current role, older than 18, or in level 5 or higher according to your reference function, read more about being a skilled worker here), and you've worked on a public holiday (see which public holidays here), you are entitled to 50% more salary on those days. So you didn't actually earned less salary this month, you just earned more than normal last month.
Your employer just switched from a previous salary administration system to Soigné
When your employer just switched to Soigné, it could be that you earn a tiny bit more or less compared to your old payslip. This has to do with how social premiums, pensions, etc are calculated. If it's more than a tiny bit, please let us know, something is probably wrong or missing.
You are missing a benefit you are supposed to get
If they just changed to Soigné, it could also be that you had a benefit that wasn't added to your new payslip yet. For example a missing travel allowance or bonus. You can check this on your payslip or ask your manager.