R中两样本t检验：合并与非合并方差结果一致的疑惑

问题重现 #

运行以下R代码：

library(oibiostat)
data("swim")
## 独立两样本合并方差t检验
t.test(swim$wet.suit.velocity, swim$swim.suit.velocity, 
       alternative = "two.sided", paired = FALSE, var.equal = TRUE)

# 不等方差两样本t检验（Welch检验）
t.test(swim$wet.suit.velocity, swim$swim.suit.velocity, 
       alternative = "two.sided", paired = FALSE, var.equal = FALSE)

得到的两个检验结果几乎一致：

合并方差t检验结果 #

Two Sample t-test

data:  swim$wet.suit.velocity and swim$swim.suit.velocity
t = 1.3688, df = 22, p-value = 0.1849
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
 -0.03992124  0.19492124
sample estimates:
mean of x mean of y 
 1.506667  1.429167 

Welch检验结果 #

Welch Two Sample t-test

data:  swim$wet.suit.velocity and swim$swim.suit.velocity
t = 1.3688, df = 21.974, p-value = 0.1849
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
 -0.03992937  0.19492937
sample estimates:
mean of x mean of y 
 1.506667  1.429167 

从理论公式来看，合并方差两样本t检验与非合并方差的Welch检验结果本应存在差异，但在swim数据集上两者输出几乎完全一致。

对比案例 #

当运行以下模拟数据的代码时，两种检验的输出结果差异明显：

set.seed(5)
x1 = rnorm(15, 95, 20)         
x2 = rnorm(50, 110, 5)
t.test(x1, x2) # Welch检验
t.test(x1, x2, var.eq=T) # 合并方差检验

手动计算验证后，Welch检验的结果是正确的，因此疑惑为何swim数据集上合并方差检验的结果会与Welch检验近乎一致，这是数据集的巧合吗？

补充说明 #

oibiostat包不在CRAN上，托管在GitHub，若未安装可运行以下代码：

devtools::install_github("OI-Biostat/oi_biostat_data")

另外无需加载包即可访问其数据集，可通过以下代码加载：

data(swim, package = "oibiostat")

内容的提问来源于stack exchange，提问作者Pai