that I noticed. By the Mishkan, is is voluntary:
דַּבֵּר אֶל-בְּנֵי יִשְׂרָאֵל, וְיִקְחוּ-לִי תְּרוּמָה: מֵאֵת כָּל-אִישׁ אֲשֶׁר יִדְּבֶנּוּ לִבּוֹ, תִּקְחוּ אֶת-תְּרוּמָתִי.
'Speak unto the children of Israel, that they take for Me an offering; of every man whose heart maketh him willing ye shall take My offering.'
Tg Yonatan stresses this, adding to the translation "and not by force."
In contrast, in the haftorah, it is mandatory, and a tax: In the haftarah: 1 Kings 5:27-28
וַיַּעַל הַמֶּלֶךְ שְׁלֹמֹה מַס, מִכָּל-יִשְׂרָאֵל; וַיְהִי הַמַּס, שְׁלֹשִׁים אֶלֶף אִישׁ.
וַיִּשְׁלָחֵם לְבָנוֹנָה, עֲשֶׂרֶת אֲלָפִים בַּחֹדֶשׁ חֲלִיפוֹת--חֹדֶשׁ יִהְיוּ בַלְּבָנוֹן, שְׁנַיִם חֳדָשִׁים בְּבֵיתוֹ; וַאֲדֹנִירָם, עַל-הַמַּס.
"And king Solomon raised a levy out of all Israel; and the levy was thirty thousand men.
And he sent them to Lebanon, ten thousand a month by courses: a month they were in Lebanon, and two months at home; and Adoniram was over the levy."
Why the difference? My idle speculation - people are more settled in a civilized country with cities and farms and the like, and would not have been so forthcoming had there not been a tax. Consider also that the people in the midbar probably did not have much else to do, or much concern for wealth or making a living, having taken the wealth of Egypt in payment for their years of servitude and not having expenditures as all their needs were taken care of by Hashem. Also, it was a much bigger venture.