Because they're in a different time zone, and Ambit shows them their time, not yours.
When you set your working hours, you're saying when you want to take calls in your time zone. When a lead opens your booking page, Ambit shows them the same slots translated to their time zone.
So if you're in New York and offer slots between 9am and 5pm, a lead in London sees those slots as 2pm to 10pm their time. A lead in Los Angeles sees them as 6am to 2pm. Same available times, different clocks.
Why it's set up this way
If you showed every visitor your time zone, leads would either book the wrong slot ("oh I thought 3pm meant my 3pm"), or they'd give up before booking. Translating times to their local zone removes that friction entirely.
What it means for the calendar invite
Once the call is booked, the calendar invite has the call's time stored in a way that respects time zones automatically. The invite shows you the call in your time zone and the lead the same call in theirs. Both are right, both refer to the same moment.
Edge cases
A few things worth knowing:
Time zone changes around daylight saving time can shift a call by an hour relative to the lead's expectation if they booked weeks in advance. The calendar handles this correctly, but you might want to send a confirmation message that mentions the time in both zones for international calls.
A lead who's traveling sees times in the time zone their device is set to. If they're between zones, they may see times that don't match either of their expected zones.
Where to go next
How do I set my working hours and minimum notice?
How do I connect Google Calendar?