Dear All,
i am working on gravity model for my master thesis : impact of Nonreciprocal preferential trade agreement on SSA
export at HS 2 digit product level to OECD countries. i am comparing the results of OLS and PPML with fixed effects and observed a positive and significant estimate for log (tariff +1). i am confused with such tariff results, can anyone help explain what is happening and why is it so, thank you.

OLS OLS PPML1 PPML
FE FE
NR-PTA 1.02*** 0.36*** 1.05*** 0.42***
(0.06) (0.05) (0.12) (0.10)
R-PTA 0.56*** 0.46*** 0.90*** 1.19***
(0.08) (0.07) (0.21) (0.10)
Tariff +1 0.12*** -0.00 0.04 0.11**
(0.02) (0.03) (0.04) (0.04)
ln_DIST -0.38*** -1.13*** -0.21** -0.64***
(0.03) (0.03) (0.09) (0.05)
Contiguity 1.67*** 0.78*** 1.02*** 0.61***
(0.12) (0.10) (0.21) (0.10)
Common Lang. 0.24*** 0.21*** -0.09 -0.13
(0.08) (0.06) (0.14) (0.08)
Colonial ties 0.61*** 0.94*** 0.08 0.53***
(0.11) (0.08) (0.12) (0.10)
ln_Y 0.55*** 0.47***
(0.01) (0.02)
ln_E 0.53*** 0.68***
(0.02) (0.04)
Constant -20.25*** 14.62*** -20.92***
(0.56) (0.31) (1.45)
Observations 59963 60507 59963 142775
Adjusted R2 0.212 0.658
Standard errors in parentheses
* p < 0.10, ** p < 0.05, *** p < 0.01