How can 1/4 Kg. of beans be Rs.15 for me and Rs.10 for the next person?

Yesterday, I went to my usual haunt to buy vegetables.

The vegetable vendor Paramasivam does his business with alacrity. He is energetic, enthusiastic, quick and above all his ability in addition is amazing. People do adore him and nobody minds a rupee or two this way or that.

I bought my share of vegetables and he charged me Rs.15 for 1/4 Kg.
By and by the day's arrival of vegetables including beans were added.

I could have asked for a replacement of my share with fresh stock which
he would have done without batting an eyebrow. But I chose not to.

Somebody pounced on the fresh stock of beans asking him what the price was.
He replied: Drumstick prices are the most today. Rs.10 apiece. Beans at 40 Rs. a kg.

I kept quiet. My theory and logic only works!! Yesterday the price could have been Rs.60 which has come down to Rs.40 today....... Anyways I bought yesterday's stock.....If I were in his shoes......