Selenium grid opens less sessions than specified in the thread-count testng xml file

Using TestNG with Selenium Grid 2.53.0 produces some unexpected behavior. The thread-count is set to 38 and the maxSessions in the grid is 100 but still got only 26 sessions open on the grid in parallel (I expect 38). Number of available nodes is 50.

echo "ext. key 3587d14f-78ea-41c7-aa64-e1bd68c665d4, ext. key 544ccb82-ae55-4ef7-9552-63dd6ba13968, ext. key f8be5689-0f3b-4649-a68f-f76731c5684f, ext. key 6f83ff7d-fa0a-4e24-84bc-f4ece7e05e06, ext. key 6e667c45-616e-4dd3-ba75-bb541eaeab7f, ext. key da278a67-1940-4f01-a0d3-e98fbfe9d614, ext. key afd9d82b-de1d-424d-b31d-2e4d1a405203, ext. key 63a72623-a11a-4e0a-afd0-c025b90e3af1, ext. key 612fde24-ef12-4347-bd40-f4f61eb80f20, ext. key a33e83a5-5575-4e65-b191-a083098f9c96, ext. key 12d81fc0-a232-4d8a-82e0-ab59253ee31c, ext. key 0ef3baad-674e-4838-b1c8-a044300b9914, ext. key 0f543047-ca7c-4807-9de9-63bb768a28c0, ext. key e341098f-f005-4e03-a927-705c42d37ce0, ext. key 746c9707-cbbd-4d18-8005-06c8aa585156, ext. key 3db3dec2-83c1-493a-87d2-2114cedf67a7, ext. key e4b64960-35f0-450d-9249-9526cee8312f, ext. key a13178a5-ee35-4640-8318-f3d2b4baf44c, ext. key 59d364fa-3a1b-4aa2-bedb-f288fd0d7d9b, ext. key f9273ec3-ae9e-48d0-81cc-3b781eb5b683, ext. key 41919e0b-d01a-45c6-aa06-ab0a3240212d, ext. key 84441daa-d673-4859-8177-ccc4a90f5fd8, ext. key f2a732d8-201d-48a8-9382-19fcf8ba5749, ext. key fb2ab9c8-78c0-419b-8a62-577920e759cb, ext. key 847892ef-16ca-49c3-86d6-74b0a2f1f5d9, ext. key 7729c3dd-8124-4cdf-b63a-e1b16e1f6178" | tr "," "\n" | wc -l
26

Can one explain what's the reson behind this behavior?

Thanks.