Possible Teretz

Perhaps we can answer the Kushya on the Rosh,

that  there is a difference between Halacha & Agada.

In Halacha The Torah tells you to count 50 days or give

40 Malkos. Torah shel bal peh tells us the number

50 & 40 are inaccurate and should be 49 & 39 there

we must say the number is inaccurate and rounded off 

to the nearest 10.

In Agadah, Chazal have more leeway to Darshen the

inaccurate number and explain it as accurate according

to Drush and not to use the explanation of rounding off.

The only problem with this answer is, the Rosh uses 

also an example of Agada. The 70 Nefashos going

down to Mitzrayim. 

The truth is there too* Chazal (Bava Basra 123b)answer 

that by counting Yocheved who was born between the 

walls (border) the number seventy is correct. (ambzy)

*see Korban Nesanel