It's like this, assume they started the peg last month, when buying rate was 198.50/- per USD.
They'll make you the offer as usual in LKR, say 250,000/-
So for everyone who joins till May, the pegged USD amount would be 250,000/198.50 ~1260USD.
This only to calculate this USD amount.
but even if you join today, your monthly pay would be calculated as 1260 * <exchage_rate_on_salary_processing_day>
assume rate was 300 by the end of this month then your salary in lkr for that month would be ~ 1260 * 300 = 378,00/-
They are doing this to give the devaluation benefit new comers, hence to attract new employees till may.
Imagine if they used the USD rate of the hiring day, which is say today, 275LKR per USD.
Then your peg would have been 250,000/275 ~ 910USD
And your salary at the end of the month with same assumptions ~ 910 * 300 = 273,000/-