The reason two samples of hydrogen can contain the same number of atoms but have different weights is due to the concept of isotopes. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons in their nuclei.
Hydrogen has three isotopes: protium (with one proton and no neutrons), deuterium (with one proton and one neutron), and tritium (with one proton and two neutrons). These isotopes have different masses due to the varying number of neutrons.
The majority of hydrogen atoms are protium, which has a relative atomic mass of approximately 1 atomic mass unit (amu). Deuterium, the second most common isotope of hydrogen, has a relative atomic mass of about 2 amu. Tritium, a radioactive isotope, has a relative atomic mass of approximately 3 amu.
When we refer to hydrogen, we usually mean protium, which has the lightest mass. However, when comparing different samples of hydrogen, it's possible for one sample to predominantly contain deuterium or tritium, which will have greater masses due to the additional neutrons.
Thus, even though two samples of hydrogen may contain the same number of atoms, if they have different isotopic compositions (different ratios of protium, deuterium, and tritium), their weights will differ.