I have no idea why they say that. Perhaps they don't understand how it works themselves? Although I'm not sure how they wouldn't but stranger things have happened. I see some wild questions asked on UC help pages on social media. It's actually both simple and incredibly complicated all at the same time! I'll try to explain it but please let me know if it sounds like utter nonsense 😂
UC is composed of multiple "elements" with set amounts for each element such as Individual/Couple, Children, Childcare, Housing (only for rent), LCWRA (people who have been medically assessed as unable to work or have limited ability to do so) Disabled Child, Carer etc. They add them all up to get the total award amount then deductions are made for income and sometimes for repayments of certain loans, debts and advance payments. The remaining amount is how much the claimant will receive that month.
Each month long period is based on circumstances and income during the claimant's assessment period. This period depends on when the first claim to UC is made. For example my assessment period is the 10th of the month to the 9th of the next month. They get info from HMRC about earnings from employment received during that period, deductions are made from the total and I then receive my payment on the 16th.
For earned income, there is something called a work allowance. This isn't a payment which is added to a claim but is the amount of income per claim that can be earned before any deductions will start to be made from the UC award. If there is no housing element on the claim the work allowance is £710. If there is housing element then work allowance is £427. Over and above these amounts, deductions are made at the rate of 55p per £1 of earned income. This is based on net income. For example, the last earned income on my claim was £2300 per month and we don't receive housing element as we have a mortgage. I don't currently work so that is my husband's salary. The calculation is £2300 - £710 which leaves £1590. Multiply this by 0.55 to get £874.50. This is how much will be deducted from the total UC amount and we get what's left.
I don't really want to share exactly what elements we receive and what our total UC is before deductions but say for example our total was £1500, we would get £625.50 after the wage deduction was made.
If my husband worked an extra day sometimes whatever extra income he received would result in a deduction of 55p in every £1 just like his regular wage. Say he got £100 for an extra day after tax/NI, they would deduct £55 from his UC total that month. But he would be £45 up overall due to the extra £100 of earned income. It eventually would of course taper down to a UC amount of zero. But, as you can hopefully see there is no sudden "cliff edge" or set earnings threshold where someone can earn a certain amount/work certain hours and if they stay just under that amount, they'll receive hundreds of pounds in benefits but work a little over that and they suddenly lose it all. It just doesn't work like that.