Federico Fazio and Benjamin Stambouli signed for Tottenham Hotspur after the season began, so there weren't many low squad numbers for them to choose from. They had to pick from what was left.
Fazio has chosen to wear No. 25. He wore No. 2 at Sevilla, but that is taken by Kyle Walker. He has worn No. 6 for Argentina before, but Vlad Chiriches already has that. So with no obvious choice, he grabbed 25, and there is no apparent reason for it.
Stambouli grabbed No. 21. He normally wears No. 22, but Nacer Chadli already has that number.
Fazio and Stambouli both figure to stick around the club for a while so they could very well have new numbers before they leave the club. Fazio, in particular, could grab No. 6, seeing as Chiriches doesn't seem long for Spurs. Chadli could be on the way out eventually, opening 22 for Stambouli, but he's first choice right now so that is still a little speculative.