I have 5 datasets, the first dataset is a baseline measurement and the other 4 datasets are post-treatment measurements at different time points . I graphed the mean and standard deviation (error bars) and I see that there is a decreasing trend in the data.

The one-way anova showed no significant difference when comparing the 5 datasets. However, I performed a two-tailed t-test to compare the baseline (dataset 1) with each post-treatment datasets ( datasets: 2,3,4 & 5) and I saw a significant difference between some datasets.

Why is that? I thought that the one-way anova results will be the same with the two-tailed t-test.

Thank you in advance for your input