az.plot_ppc的观测曲线与实际数据的kde图不匹配

Question

我正在建模我认为是伽马分布的数据，所以我编写了一个PyMC3模型，如下所示

import pymc3 as pm
import arviz as az

with pm.Model() as model2:
    alpha = pm.Exponential('alpha', 5)
    beta = pm.Exponential('beta', 5)
    
    p_factor = pm.Gamma('p_factor', alpha = alpha, beta = beta, observed = d.productivity_factor)
    
    prior_checks = pm.sample_prior_predictive(samples=100, random_seed= seed)
    
    trace2 = pm.sample(2000, tune=2000)

然后我绘制一个后验预测检查

with model2:
    ppc2 = pm.sample_posterior_predictive(trace2, var_names=["alpha", "beta",'p_factor'], random_seed=seed)
    
idata = az.from_pymc3(trace2, posterior_predictive=ppc2)
az.plot_ppc(idata)

但观察到的曲线与我的预期不符。我做错什么了吗