Im getting a different output than expected when using df.loc to change some values of the df

I have a data frame, and I want to assign a quartile number based on the quartile variable, which gives me the ranges that I later use in the for. The problem is that instead of just changing the quartile number, it its creating n (len of the datframe) rows, and then using the row number for the loop.

expected result actual output

quartile = numpy.quantile(pivot['AHT'], [0.25,0.5,0.75])
pivot['Quartile'] = 0

for i in range(0,len(pivot)-1):

if i <= quartile[0]:
    pivot.loc[i,'Quartile'] = 1
    

elif i <= quartile[1]:
    pivot.loc[i,'Quartile'] = 2


elif i <= quartile[2]:
    pivot.loc[i,'Quartile'] = 3
    
    
else:
    pivot.loc[i,'Quartile'] = 4