QuantLib FixedRateBond versus Excel Yield()

Question

I'm currently trying to calculate the Yield of Treasury Bonds in Python using the QuantLib library. As a reference I used the Excel Yield function, but I'm getting different results for the same input-values.

I don't understand why I'm getting different results and I don't understand where this difference comes from.

I have two columns using the Yield function, a YTM column and an Actual Yield column. I'll start with YTM.

YTM

In Excel I use the following values:

=YIELD(valuedate, maturitydate, rate, price, redemption, coupons, 3)

• valuedate = 26/11/2021
• maturitydate = 26/01/2026
• rate = 0.000%
• price = 45.340
• redemption = 100
• coupons = 1
• dayCount = 3 = Actual365

With these values I get a YTM of 20.903%.

To replicate this in Python I used this code:.

#Dates
start = 26/11/2021 
maturity = 26/01/2026

#Yield Params
redemption = 100
coup = 1
price = 45.340
couponRate = 0.000%
rate = [couponRate]
calendar = ql.NullCalendar()
settlementDays = 0

tenor = ql.Period(ql.Annual)                   
fixedRateBond = ql.FixedRateBond(settlementDays, calendar, redemption, start, maturity, tenor, rate, ql.Actual365Fixed())               
fixedRateBond.bondYield(price,
                        ql.Actual365Fixed(),
                        ql.SimpleThenCompounded, #SimpleThenCompounded gives best result yet
                        ql.Annual,
                        start, 
                        1.0e-16, 
                        100
                        )

With QuantLib, using the same values as Excel Yield, I get a Yield of 20.827% which is a small difference of 0.075%.

But, when looking at Actual Yield, I get bigger differences using the same parameters as with YTM.

So, with the same mindset of trial-and-error, I've used slightly different parameters which returned the best results I could obtain.

Actual Yield

Excel values:

=YIELD(quotationdate+3, maturitydate, rate, quotationprice, redemption, coupons, 3)  

(note: that we do +3 because the accrued interest is calculated on the settlementDate)

• quotationdate = 2/3/2022
• maturitydate = 16/10/2049
• rate = 3.897%
• quotationprice = 15
• redemption = 100
• coupons = 1
• dayCount = 3 = Actual365

which returns a yield of 26.044%

QuantLib:

ql.Settings.instance().evaluationDate = 2/3/2022
start = 2/3/2022
maturity = 16/10/2049

coup = 1
price = 15
couponRate = 3.897%
rate = [couponRate]
settlementDays = 3

calendar = ql.NullCalendar()
businessConvention = ql.Unadjusted
dateGeneration = ql.DateGeneration.Backward
monthEnd = False
redemption = 100
tenor = ql.Period(ql.Annual)

schedule = ql.Schedule(start, 
                       maturity, 
                       tenor, 
                       calendar, 
                       businessConvention,
                       businessConvention, 
                       dateGeneration, 
                       monthEnd)
                
 bond = ql.FixedRateBond(settlementDays, redemption, schedule, rate, ql.Actual365Fixed())
 bond.bondYield(price,
                ql.Actual365Fixed(),
                ql.Compounded,
                ql.Annual)

Resulting in a Yield of 26.368% which is a difference of 0.324%

I want to stress that I don't have a financial background. I got these parameters via other similar StackOverflow questions and through a lot of trial-and-error.

Using the same parameters in QuantLib for Actual Yield as used in YTM gave worse results than these parameters.

I've consulted every StackOverflow question about bondYield but can't seem to find an answer to my question(s).

Could someone explain to me which parameters I should use to replicate the Excel Yield() function and why I'm getting differences between QuantLib and Excel?

Also, is the Excel function granted to be 100% correct?